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 Israel's air strike on a Syrian air base on Monday that killed several Iranians "will not remain unanswered".
Iran's semi-official 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 pre-dawn 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 air strike by US ally Israel have escalated tensions in the already volatile Middle East 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 Donald 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".
Many dead, including women and children, in mindless CHEMICAL attack in Syria. Area of atrocity is in lockdown and encircled by Syrian Army, making it completely inaccessible to outside world. President Putin, Russia and Iran are responsible for backing Animal Assad. Big price...— Donald J. Trump (@realDonaldTrump) April 8, 2018
Syrian opposition activists said 40 people died in Saturday night's chemical attack in Douma, the last remaining rebel bastion just outside of Damascus.
The opposition blamed Assad's forces for the attack, accusations the government and its Russian backers strongly deny.
UN secretary-general Antonio Guterres said he was outraged by Syria's apparent chemical weapons use against civilians.
If confirmed, the use of such weapons would be a violation of international law, Mr Guterres said in a statement.
Mr Guterres also said he reaffirms 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.
- Digital Desk and Press Association