Four Iranian military personnel were killed in an airstrike on a Syrian airbase on Sunday, Iran's Fars news agency said, as Syria and its main allies Iran and Russia blamed Israel for carrying out the attack.

Israel has not confirmed or denied mounting the raid, but Israeli officials said the Tiyas, or T4, airbase near Homs was being used by troops from Iran, and that Israel would not accept such a presence in Syria by its arch foe.

The Revolutionary Guards, Iran's most powerful military force, has been fighting in support of Syrian President Bashar al-Assad for several years. More than 1,000 Iranians have been killed in Syria, including senior members of the Guards.

The semi-official Fars news said on Monday that four "defenders of the shrine" were killed in the airstrike.

Iran calls its fighters in Syria the defenders of the shrine, as it says the forces are there to protect the Zeinab Shrine, a Shia holy site near Damascus.

Drone operator killed?

Some Iranian media reported that one of those killed was a member of the Revolutionary Guards Aerospace Force, a division that operates drones.

Israel said in February it had intercepted an Iranian drone launched from Syria, and carried out airstrikes against air defences and Iranian targets in the country. Iran denied at the time the downing of its drone in Israel.

The Syrian Observatory for Human Rights, a British-based monitor, said at least 14 people were killed in the Sunday's raid, including some fighters of various nationalities.

Iranian Foreign Ministry spokesperson Bahram Qasemi condemned the attack, calling it "an obvious breach of international laws that would make the Syrian crisis and regional equations more complicated."

The attack took place hours after U.S. President Donald Trump warned of a "big price to pay" following the reports of a poison gas attack on the rebel-held town of Douma that killed dozens of people, including children.