WASHINGTON -- Thousands of Islamic State foreign fighters and family members have escaped the American-led military campaign in eastern Syria, according to new classified American and other Western military and intelligence assessments, a flow that threatens to tarnish American declarations that the militant group has been largely defeated.
As many of the fighters flee unfettered to the south and west through Syrian Army lines, some have gone into hiding near Damascus, the Syrian capital, and in the country's northwest, awaiting orders sent by insurgent leaders on encrypted communications channels.
Other battle-hardened militants, some with training in chemical weapons, are defecting to Al Qaeda's branch in Syria. Others are paying smugglers tens of thousands of dollars to spirit them across the border to Turkey, with an eventual goal of returning home to European countries.
The sobering assessments come despite a concerted effort to encircle and "annihilate" -- as Defense Secretary Jim Mattis put it -- Islamic State fighters in Raqqa, the group's self-proclaimed capital, which fell in the fall, and pursue other insurgents who fled south into the Euphrates River Valley toward the border with Iraq.
"ISIS fighters are fleeing Syria and Iraq," the homeland security secretary, Kirstjen Nielsen, said in remarks in Washington last week. "Jihadists are going underground, dispersing to other safe havens, including on the internet, and returning to their home countries."
Gen. Paul J. Selva, the vice chairman of the Joint Chiefs of Staff, told reporters last week that the remaining Islamic State leadership, even while on the run, still had "fairly robust" communications with its shadowy network of fighters now on the lam.
While President Trump highlighted the liberation of almost all of the Islamic State's territory in Iraq and Syria in his State of the Union address last week, American military and intelligence officials say the group is still able to inspire and enable followers to carry out attacks. Mr. Trump seemed to acknowledged this in his speech, noting, "There is much more work to be done."
Analysts say they are also seeing signs that Islamic State fighters are adopting guerrilla tactics to terrorize civilians.
"The group is transitioning into an underground organization that places more weight on asymmetric tactics, like suicide bombings against soft targets in government-secured areas like Baghdad," said Otso Iho, a senior analyst at Jane's Terrorism and Insurgency Center at IHS Markit in London.
Mr. Iho cited an attack by two suicide bombers in Baghdad last month that killed three dozen people and injured 90 more. The attack took place in a busy Baghdad square where day laborers gather to look for work.
Estimates of how many fighters may have escaped into the deserts of Syria or Iraq and beyond are difficult to pin down, but American and other Western intelligence and counterterrorism analysts with access to classified assessments put the number in the low thousands. Many are traveling with spouses and children who are likely to have been radicalized during more than three years of Islamic State control of the region and could pose security risks as well, analysts say.
In December, Col. Ryan Dillon, the chief spokesman for the American-led military campaign in Iraq and Syria, said in a briefing with Pentagon reporters: "Syrian regime commanders in eastern Syria suggest that ISIS fighters" from the Middle Euphrates River Valley "may have slipped through porous Syrian and Russian defenses to arrive in areas near Damascus."
Asked late last month by The New York Times about indications that as many as 1,000 fighters and family members had fled the Euphrates River area just in recent days, Colonel Dillon's command replied in a statement: "We know that the Syrian regime has given ISIS the leeway to travel through their area of operations, but we cannot confirm any alleged incidents or operations that are taking place outside our area of operations."
The United States military is concerned that a Turkish offensive against the Kurdish-dominated Syrian Democratic Forces in Afrin, in northern Syria, has worsened the problem. The S.D.F. has been working with the Americans in former Islamic State-held areas to interdict fleeing jihadists, but those efforts have been greatly reduced as the Kurds have shifted resources to reinforce Afrin.
Mustafa Balli, a spokesman for the Syrian Democratic Forces headquarters, blamed the Turkish assault on Afrin for what he said was the Islamic State's resurgence.
"Since this invasion of Afrin by Turkey, ISIS is getting stronger in the south," he said. "The battle against ISIS in the south, and the Turks in Afrin, is the same battle. The Turks want to give another chance to ISIS to grow again. Before the Turkish invasion, we were very close to finishing ISIS."
Some 40,000 fighters from more than 120 countries poured into the battles in Syria and Iraq over the past four years, American and other Western officials say. While thousands died on the battlefield, officials say many thousands more probably survived to slip away to conflicts in Libya, Yemen or the Philippines, or have gone into hiding in countries like Turkey. About 295 Americans are believed to have traveled to Iraq or Syria, or tried to, American officials said.
Of more than 5,000 Europeans who joined those ranks, as many as 1,500 have returned home, including many women and children, and most of the rest are dead or still fighting, according to Gilles de Kerchove, the European Union's top counterterrorism official.
"The thought that these foreign fighters who have participated in this fight now for over two years will quietly leave Syria and return to their jobs as shopkeepers in Paris, in Brussels, in Copenhagen, is ludicrous," said General Selva, the vice chairman of the Joint Chiefs. "That's a very compelling problem."
Still, the number of Islamic State fighters returning home to Europe and North Africa has been much smaller than anticipated, counterterrorism officials say. That is in part because the Trump administration intensified its focus on preventing fighters from seeping out of Raqqa and Mosul, their former stronghold in Iraq, and more militants fought to the death than expected. Hundreds also surrendered in Raqqa.
Hundreds of others have been captured and are being held by American-backed Kurdish militias in northern Syria, raising fears among United States military officials of potentially creating a breeding ground for extremists -- repeating a key security mistake of the Iraq war.
But the new assessments, bolstered by reports from analysts and smugglers in the region, suggest that Islamic State fighters are fleeing to more hospitable parts of Syria and Iraq, or to third countries where they can lie low.
Beyond the recent suicide bombings in Baghdad, a major American airstrike last month demonstrates the Islamic State's continued resiliency and threat, military officials said.
Armed Reaper drones and Navy F/A-18 fighters from the aircraft carrier Theodore Roosevelt carried out a strike on Jan. 20 in Al Shafah, Syria, in the Euphrates River Valley, that killed about 150 fighters, the American military said. The strike, one of the largest single aerial assaults against the Islamic State in three years, was based on intelligence collected over about a week. The strike hit two large buildings that were used as a command headquarters and a media distribution center, military officials said.
The size and concentration of fighters took American officials by surprise. "The ISIS headquarters contained a heavy concentration of ISIS fighters who appear to have been massing for movement," Maj. Gen. James B. Jarrard, commanding general for Special Operations forces in Iraq and Syria, said in a statement.
"ISIS continues to demonstrate the ability to mass large numbers in its attempt to retain a stronghold in Syria," the American-led command in Iraq said in the same statement.
Ahmad Ramadan, the head of the Euphrates Center Against Violence and Terrorism in Istanbul, said that the Islamic State was still present in many villages east of the Euphrates River -- the informal demarcation line between Russian-backed Syrian troops to the west and American-backed Syrian militias to the east. "ISIS nowadays are spreading all over Syria," he said via Facebook chat.
Government and independent analysts in Syria and in Washington, including the Institute for the Study of War, said there was a thriving trade in smuggling Islamic State fighters across the border into Turkey, where intelligence officials believe they are linking with clandestine cells.
According to the independent British-based Syrian Observatory for Human Rights, senior Islamic State operatives from Raqqa and Deir al-Zour, in the Euphrates River Valley, have paid bribes of $20,000 to $30,000 for safe passage into Turkey.
"I smuggled about 50 ISIS fighters into Turkey," said Abu Omar, a smuggler between Syria and Turkey, adding that they were a mix of Syrian and foreign fighters, often disguised in women's clothes to help elude Turkish border patrols.
Abu Omar added that the number of fleeing Islamic State fighters and senior leaders, including many foreigners, increased over the summer when the American-backed offensive against Raqqa began.
"I was really shocked when I saw them," he said in a WhatsApp message. "They were wearing cool clothes, classic jeans with many necklaces, trying to disguise as much as they can. They hid their passports in their boots. They were completely shaved; you never guess they are ISIS. They didn't speak any Arabic, few words."
Hwaida Saad contributed reporting from Beirut, Lebanon, Rod Nordland from Kobani, Syria, and Karam Shoumali from Berlin.