Nine pro-Iranian militiamen were killed when Israeli planes fired several missiles in central Syria, according to a group monitoring the Syrian conflict.
The Britain-based Syrian Observatory for Human Rights said the strikes late on April 21 targeted "military posts for Iranian militias" in the Palmyra desert in Homs Province.
The Israeli military declined to comment.
The slain militiamen included three Syrian nationals and six foreigners of unknown nationalities, the observatory said, adding that the dead included some fighters loyal to the Lebanese militant group Hezbollah.
Syrian state news agency SANA reported that the country's air defenses "confronted an Israeli missile aggression" and "shot down a number of the hostile missiles before reaching their targets."
Syrian state media regularly overstates the success of the country's defenses against Israeli air strikes.
Israel has carried out hundreds of strikes in Syria in recent years targeting Iranian forces, Syrian government positions, and allied militias.
Western and Israeli intelligence say Iranian forces and allied militias use the eastern Syrian desert bordering Iraq to transfer fighters and advanced weapons systems to support the Syrian government and the Hezbollah group.
Last week, an Israeli drone fired two missiles at a vehicle carrying Hezbollah members in Syria close to the border with Lebanon. The Shi'ite militant group said its members escaped.
The strikes on April 20 came as Iranian Foreign Minister Mohammad Javad Zarif met in Damascus with Syrian President Bashar al-Assad.
Along with Russia, Tehran has provided crucial military support to Assad during the country's civil war, which entered its 10th year last month.
More than 400,000 people have been killed and millions displaced since the conflict began with a crackdown on anti-government protesters in March 2011.