Overnight strikes on positions held by Iranian-backed militias and their allies in eastern Syria killed 14 fighters, a war monitor said Tuesday.
It was not immediately clear who carried out the strikes in the desert near the town of Mayadin, which came minutes after Syrian air defenses intercepted Israeli strikes over the north of the country, the Britain-based Syrian Observatory for Human Rights said.
A spokesman for the U.S.-led coalition battling the Daesh terrorist group said it was not responsible for the strikes.
Observatory head Rami Abdul Rahman said it was "likely" that Israel mounted the operation, which killed several Iraqi as well as Iranian fighters.
State media did not report the strikes.
Iranian-backed militias and their allies command a significant presence in eastern Syria south of the Euphrates Valley. The region lies close to the Iraqi border.
Israel has launched hundreds of strikes in Syria since the start of the civil war in 2011, targeting regime troops, allied Iranian forces and Hezbollah fighters.
It rarely confirms details of its operations in Syria but says Iran's presence in support of Bashar Assad is a threat and that it will continue its strikes.
Just before midnight on Monday, Syrian air defenses intercepted Israeli missiles targeting a research facility in Aleppo province, state media said.
State news agency SANA said the intercepted missiles targeted several "military depots" in the Al-Safira area southeast of Aleppo, Syria's second-largest regime-held city.
On Friday, Israeli forces hit a missile depot in central Syria used by Lebanese militant group Hezbollah, another Iran ally, hours after Israeli helicopters fired missiles at other targets in southern Syria.