(AP) -- Syrian government forces and their allies were on alert and taking precautionary measures Tuesday at military bases and posts in government-held areas across the country amid fears of a U.S. strike in the aftermath of an apparent chemical weapons attack in Syria, war monitors said.
The measures came as a senior Iranian official visiting Damascus warned that Israel's airstrike on a Syrian air base Monday that killed several Iranians "will not remain unanswered."
Iran's semiofficial Tasnim news agency put the number of Iranians killed in the attack at seven. It said the victims have been transferred to the Iranian capital, Tehran, and that funerals will be held in the victims' cities of origin across the country.
Russia and the Syrian military blamed Monday's predawn strike, which reportedly killed 14 people, on Israel. There has been no comment from Israel on the strike on the T4 air base in Syria's central Homs province.
The suspected poison gas attack in the Syrian rebel-held town of Douma, just east of Damascus, and Monday's airstrike by U.S. ally Israel have escalated tensions in the already volatile Mideast and raised the threat of possibly imminent American retaliation.
Ali Akbar Velayati, an aide to Iran's supreme leader, issued his stark warning upon arrival in the Syrian capital on Tuesday. He was quoted by the country's state-run news agency, IRNA.
Iran is one of President Bashar Assad's strongest backers and has sent thousands of Iran-backed gunmen to back his forces.
President Trump has been threatening a military strike against Syria, vowing to respond "forcefully" to Saturday's apparent chemical weapons attack on civilians in Douma, warning that Russia -- or any other nation found to share responsibility -- will "pay a price."
Syrian opposition activists said 40 people died in Saturday night's chemical attack in Douma, the last remaining rebel bastion just outside Damascus. The opposition blamed Assad's forces for the attack, accusations the government and its Russian backers strongly deny.
U.N. Secretary-General Antonio Guterres said Tuesday he is outraged by Syria's apparent chemical weapons use against civilians. If confirmed, the use of such weapons would be a violation of international law, Guterres said in a statement.
Guterres also said he reaffirms his support of an investigation into the alleged attack by the Organization for the Prohibition of Chemical Weapons.
A senior Russian lawmaker said Tuesday that Moscow is willing to help arrange a visit for experts from the international chemical weapons watchdog to the site of the suspected poison gas attack in Douma.
Asked in Berlin whether she supports the idea, German Chancellor Angela Merkel responded unenthusiastically, saying that "the evidence that chemical weapons were used there is very, very clear."