Separately, the Syrian defence ministry said an Israeli strike on army positions elsewhere in the province had wounded two soldiers late on Monday.

"Six pro-Iranian fighters were killed in a probable Israeli strike" on Monday evening, the Syrian Observatory for Human Rights said.

Israeli strikes targeted "three sites belonging to Iran-backed groups" close to the border city of Albu Kamal, the Britain-based monitor said.

Militias linked to Iran's Revolutionary Guards have a heavy presence across Syria, especially around the border with Iraq.

During more than a decade of conflict in Syria, neighbouring Israel has launched hundreds of air strikes on its territory, targeting Iran-backed forces and Hezbollah fighters as well as Syrian army positions.

Separately, the Syrian defence ministry said that shortly before midnight (2100 GMT) on Monday, an Israeli air strike had wounded two soldiers near the city of Deir Ezzor.

"The Israeli enemy carried out air strikes on some of our armed forces' positions near the city of Deir Ezzor," leaving "two soldiers wounded," a ministry statement said.

Israel rarely comments on individual strikes it carries out on targets in Syria, but it has said repeatedly that it will not allow its arch foe Iran to expand its presence.

Last month, Israeli air strikes killed two soldiers on Syria's Mediterranean coast, state media said.

In March, US strikes on Iran-linked groups in eastern Syria killed 19 people, including both Iran-backed fighters and Syrian soldiers, the Observatory said.

The war in Syria has killed more than half a million people since it broke out in 2011, following the bloody repression of pro-democracy protests.

It quickly escalated into a broader conflict that pulled in jihadists and foreign powers.