SYRIAN government forces and their allies are on alert and taking precautionary measures at military bases and posts in government-held areas across the country amid fears of a US strike in the aftermath of an apparent chemical weapons attack, war monitors said.

The measures came as a senior Iranian official visiting Damascus warned that the air strike on a Syrian air base on Monday, which reportedly killed 14 people, “will not remain unanswered”.

Russia and the Syrian military blamed the pre-dawn strike on the T4 air base in Homs province, on Israel.

The air strike and the suspected poison gas attack in the rebel-held town of Douma have escalated tensions in the area and raised the threat of US retaliation.

Iran, one of President Bashar Assad’s strongest backers, has reportedly sent thousands of gunmen to back his forces.

US President Donald Trump has vowing to respond “forcefully” to Saturday’s apparent chemical weapons, warning that Russia – or any other nation found to share responsibility – will “pay a price”.

UN secretary-general Antonio Guterres said he was outraged by the apparent chemical weapons attack and reaffirmed his support of an investigation into the alleged attack by the Organisation for the Prohibition of Chemical Weapons.

A senior Russian politician said 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.

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”.

Swedish foreign minister Margot Wallstrom dismissed the idea of military retaliation on Tuesday, saying “it’s the role of the Security Council first of all to advocate a non-violent resolution to this”, to avoid escalation.

The Britain-based Syrian Observatory for Human Rights said that as of Monday night, troops have been on a 72-hour alert and are fortifying their positions.

The observatory’s chief Rami Abdurrahman said the alert includes all army positions and bases from the southern province of Sweida all the way to Aleppo province in the north, the Mediterranean coast in the west to Deir el-Zour province in the east, along the Iraqi border.

The Sound and Picture Organisation, an activist collective in eastern Syria, said that Iranian fighters and members of Lebanon’s Hezbollah group have evacuated their positions in the Boukamal area, near Iraq’s border.

A Lebanese politician with close links to the Syrian government confirmed that precautionary measures were being taken throughout Syria.

The politician said the atmosphere and American statements hint of a possible strike “but there are no indications on the ground so far”.

The politician said the Syrian government was expecting a limited strike, similar to the one on Shayrat air base last year, referring to last year’s US attack in the central province of Homs following a chemical weapons attack in northern Syria that Washington blamed on the government.