An early-morning strike on a Syrian government air base in the country's centre killed 14 fighters, including Iranian forces allied to the regime, a monitoring group said on Monday.
"At least 14 fighters were killed in the strike on the T-4 airport, among them Iranian forces," said the Syrian Observatory for Human Rights, a Britain-based war monitor.
The T-4 base lies in Syria's central Homs province and is also known as the Tiyas airport.
Forces from regime allies Russia and Iran, as well as fighters from the Tehran-backed Lebanese Hezbollah militia, are known to have a presence there, said Observatory chief Rami Abdel Rahman.
Syria and its main ally Russia blamed Israel for carrying out the attack.
Israel, which has struck Syrian army locations many times in the course of its neighbour's seven-year-old civil war, has not confirmed nor denied mounting the raid.
But Israeli officials said the Tiyas, or T-4, air base was being used by troops from Iran and that Israel would not accept such a presence in Syria by its arch foe.
The attack demonstrated the multi-faceted nature of the Syria conflict, which started in March 2011 as an uprising against Assad and now involves several countries and various insurgent groups in a web of alliances.
It took place hours after US 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 which killed dozens of people, including children.
Trump referred in a Tweet to "Animal Assad" and criticised Russia and Iran for backing the Syrian leader, directly naming Russian President Vladimir Putin.
Damascus denied its forces had launched any chemical assault and Russian Foreign Minister Sergei Lavrov said such allegations were false and a provocation. Lavrov also said the strike on the T-4 base was a dangerous development.
Syrian state TV initially said the United States was suspected of carrying out the attack on T-4. Washington denied this, and France, which in February had said it would strike in the event of a chemical weapon attack on civilians by Syrian government forces, also said its forces were not involved.
The Russian military, whose support for Assad has turned the war in his favour, said two Israeli F-15 war planes carried out the strike. Interfax news agency cited the Russian Defence Ministry as saying Syrian air defence systems had shot down five of eight missiles fired.
Syrian state media, citing a military source, carried a similar report. "The Israeli aggression on the T4 airport was carried out with F-15 planes that fired several missiles from above Lebanese land," state news agency SANA said.