Those killed were "Iraqi paramilitary fighters loyal to Iran, seven of whom were killed outside the city of Mayadeen," Rami Abdul Rahman, head of the Britain-based Syrian Observatory for Human Rights, said.

The other nine were killed in strikes south of the city of Albu Kamal, on the Iraqi border further east.

Abdul Rahman said Israel was "likely" responsible for the attack, which if confirmed, would mark the second such strike in less than 24 hours and the third this week.

Late Wednesday, Syrian air defences intercepted missiles fired by an Israeli warplane at the T4 air base in central Syria, state news agency SANA reported.

"Our air defences intercepted most of them," it said, adding the attack caused only material damage.

On Monday, Israeli strikes killed one civilian, three government troops and seven allied foreign fighters, the Observatory said.

Monday's strikes hit Syrian army positions south of Damascus and facilities used by Iran-backed paramilitaries, including fighters of Lebanon's Hezbollah movement, in the southern province of Daraa, the Observatory said.

Israel has carried out hundreds of air and missile strikes on Syria since the civil war broke out in 2011, targeting Iranian and Hezbollah forces as well as government troops.

The Israeli army rarely acknowledges individual strikes, but did confirm on August 3 that it had used fighter jets, attack helicopters and other aircraft to hit military targets in southern Syria.