LONDON -- Saudi Arabia charged Monday that a missile fired at its capital from Yemen over the weekend was an "act of war" by Iran, in the sharpest escalation in nearly three decades of mounting hostility between the two regional rivals.
"We see this as an act of war," the Saudi foreign minister, Adel Jubair, said in an interview on CNN. "Iran cannot lob missiles at Saudi cities and towns and expect us not to take steps."
The accusation, which Iran denied, came a day after a wave of arrests in Saudi Arabia that appeared to complete the consolidation of power by the crown prince, Mohammed bin Salman, 32. Taken together, the two actions signaled a new aggressiveness by the prince both at home and abroad, as well as a new and more dangerous stage in the Saudi cold war with Iran for dominance in the region.
"Today confrontation is the name of the game," said Joseph A. Kechichian, a scholar at the King Faisal Center for Research and Islamic Studies in Riyadh, Saudi Arabia, who is close to the royal family. "This young man, Prince Mohammed bin Salman, is not willing to roll over and play dead. If you challenge him, he is saying, he is going to respond."
The accusations raise the threat of a direct military clash between the two regional heavyweights at a time when they are already fighting proxy wars in Yemen and Syria, as well as battles for political power in Iraq and Lebanon. By the end of the day Monday, a Saudi minister was accusing Lebanon of declaring war against Saudi Arabia as well.
Even before the launching of the missile on Saturday, which was intercepted en route to Riyadh, the Saudi capital, the crown prince had staged another surprise demonstration of the kingdom's newly aggressive posture toward Iran and Lebanon. The prince hosted a visit from Saudi Arabia's chief Lebanese client, Prime Minister Saad Hariri, who stunned the region by announcing his resignation, via video from Riyadh, in protest against Iran's undue influence in Lebanese politics.
Even some of Mr. Hariri's rivals speculated that his Saudi sponsors had pressured him into the statement. Hassan Nasrallah, the leader of Hezbollah, the Iranian-backed Lebanese militia, said over the weekend that the Saudis had all but kidnapped Mr. Hariri. Mr. Nasrallah urged Mr. Hariri to return to Lebanon for power-sharing talks "if he is allowed to come back."
On Monday, Saudi Arabia released a photograph of Mr. Hariri meeting with King Salman that was widely seen as an effort to contradict the theory that the prime minister was effectively a hostage.
The Saudi claims that Iran had provided the missile could not be independently verified.
Mr. Jubair, the foreign minister, said the missile had been smuggled into Yemen in parts, assembled in Yemen by operatives from Hezbollah and the Islamic Revolutionary Guards Corps of Iran, and fired from Yemen by Hezbollah.
A statement from the Saudi Arabian news agency said "experts in military technology" had determined from the remains of that missile and one launched in July that both had come from Iran "for the purpose of attacking the kingdom."
Citing allegations of Hezbollah's role, Thamer al-Sabhan, minister of state for Persian Gulf affairs, said Monday that Saudi Arabia considered the missile attack an act of war by Lebanon as well.
"We will treat the government of Lebanon as a government declaring a war because of Hezbollah militias," Mr. Sabhan told the Saudi-controlled Al Arabiya network. "Lebanon is kidnapped by the militias of Hezbollah and behind it is Iran."
The top commander of the Islamic Revolutionary Guards Corps in Iran called the accusation of Iranian involvement in the missile attack "baseless."
"These missiles were produced by the Yemenis and their military industry," the commander, Maj. Gen. Mohammad Ali Jafari, told the semiofficial news agency Tasnim.
Iran's foreign minister, Mohammad Javad Zarif, accused Saudi Arabia of "wars of aggression, regional bullying, destabilizing behavior & risky provocations," in a statement on Twitter. Saudi Arabia "bombs Yemen to smithereens, killing 1000's of innocents including babies, spreads cholera and famine, but of course blames Iran," Mr. Zarif said.
American officials have previously accused Iran of arming its Yemeni allies, the Houthis. But it was unclear how Saudi experts would know how or why Iran provided the missiles.
With help from allies including the United Arab Emirates and the United States, Saudi Arabia has enforced a sea and air blockade around Yemen since it launched an attack on the Houthi forces there more than two years ago.
In Saudi Arabia, Prince Mohammed's grip on power tightened further on Monday. American officials tracking the situation said that as many as 500 people, including at least 11 princes, had been rounded up in a wave arrests directed by Prince Mohammed in the name of a crackdown on corruption.
Some are expected to face undisclosed criminal charges while others may be pressured to testify about their roles in corruption schemes. Men were sleeping on mats on the floor of the lobby of the Ritz-Carlton, which has been transformed into a singularly luxurious prison to house the detainees.
The arrests cemented Prince Mohammed's dominance over military, foreign, internal security, economic and social affairs inside the kingdom, freeing him to pursue an aggressive confrontation with Iran. That posture has been a hallmark of his rise over the two and a half years since his father, King Salman, 81, took the throne.
Prince Mohammed has sharply escalated a cold war with Iran, stepping up Saudi Arabia's efforts to push back Iranian influence in the Syrian civil war, plunging the kingdom into a protracted military conflict against Iranian-allied forces in Yemen, and isolating neighboring Qatar in part for being too close to Iran.
His hawkish stance toward Iran also appears to have formed the basis for a close bond with President Trump, who visited Riyadh this year and maintained a conspicuous silence over the weekend about Prince Mohammed's campaign of arrests. On Tuesday while traveling in Asia, he praised the arrests, saying the king and crown prince "know exactly what they are doing."
Mr. Trump's adviser and son-in-law, Jared Kushner, recently left Riyadh after his third visit this year. Prince Mohammed and Mr. Kushner stayed up talking together until the small hours of the morning at a ranch in the desert, according to an American official briefed on the trip.
Mr. Kechichian, of the King Faisal Center, said the arrests and the confrontation with Iran represented the convergence of two long-term agendas for Prince Mohammed.
"Inside he has been able to put his men into positions of influence and he has pushed aside his rivals," Mr. Kechichian said. "And ever since President Trump's visit to Riyadh there has been a very consistent policy with the essential coordination of the United States, and Iran is in the bull's-eye."
Robert Jordan, a former American ambassador to Saudi Arabia who now practices law in the region, said Prince Mohammed's aggression was "compounded somewhat by what people would call a green light from President Trump."
Mr. Trump has encouraged Saudi Arabia and its allies "to be more forceful against Iran, and to take more charge of their own neighborhood, and they have taken that to heart," Mr. Jordan said. "They know America will have their back."
Saudi Arabia also said Monday that it would "temporarily" close Yemen's land, sea and air ports of entry in response to the missile firing, in order to tighten inspections and stop any weapons shipments. It pledged to provide for "the continuation of the entry and exit of humanitarian supplies and crews."
However, the United Nations said that two aid flights scheduled for Monday had not been allowed to depart for Yemen.
"We're trying to see whether we can get our normal access restored," Farhan Haq, a United Nations spokesman, said at a daily briefing. "We underscored to all parties the need for regular humanitarian access."
The United Nations considers Yemen, the Middle East's poorest country, one of the world's biggest humanitarian emergencies. Roughly 17 million people -- 60 percent of the population -- need food assistance, and seven million are at risk of famine. Nearly 900,000 Yemenis have been sickened by cholera.
Saudi Arabia accompanied its accusations against Iran with the announcement that it would pay bounties of up to $30 million for information leading to the capture of 40 Houthi leaders in Yemen.
"We fear nothing," one leader on the list, Mohammad Ali al-Houthi, said in a defiant speech on Monday in the Yemeni capital, Sana.
He called the arrests ordered by Prince Mohammed "a coup leading to the throne" and invited any dissident Saudis to take refuge in Yemen. "We tell the citizens and princes in Saudi Arabia that the Yemeni people are opening their arms to you. None will endure injustice."
Yemen's Houthi-controlled Defense Ministry said over the weekend that its forces had targeted Riyadh's airport with a long-range missile. Immediately after the firing, the Saudi-led coalition hit Sana with the heaviest barrage of airstrikes in more than a year.
With the support of Iran, the Houthis overthrew the internationally recognized government of President Abdu Rabbu Mansour Hadi in early 2015, and they have controlled much of the country since.
While the Houthis have long had loose ties to Iran and have received some support, there has never been proof that they were proxies under the direct command of Tehran, as the Saudis assert, analysts say. But the Saudi intervention appears to have unintentionally brought the Houthis and Iranians closer together, Mr. Jordan, the former ambassador, noted.
The Saudi claim about Iran's responsibility for the missile attack was difficult to evaluate in part because of the long and complicated history of illicit weapons shipments to Yemen.
South Yemeni forces acquired Soviet missiles during their civil war with what was then North Yemen before it ended in 1994, and the subsequent national government of Yemen, whose institutions are now under the control of the Houthi faction, had said as long ago as 2002 that it had bought a shipment of Scud missiles from North Korea.
State Department cables published by WikiLeaks indicate that Yemen had resumed buying North Korean missiles as recently as 2009. But the Houthi alliance with Iran makes it impossible to rule out the possibility that Tehran provided or procured the missiles, even if they were manufactured in North Korea.
Analysts at IHS Jane's say that it would be difficult for Iran to ship whole missiles to Yemen, but that the missiles could have been acquired from North Korea before the current conflict started.
Riyadh has been attacked twice before with missiles from Yemen, in February and March. The Saudi border area, including military bases in the southern city of Jizan, has also been targeted several times.
Reporting was contributed by Shuaib Almosawa from Sana, Yemen; Thomas Erdbrink from Tehran; Anne Barnard from Beirut, Lebanon; Rick Gladstone from New York; and Eric Schmitt from Washington.