The strikes targeted "three positions of the (Iranian) Revolutionary Guards and allied (Iraqi) militias" in Albu Kamal, in the Euphrates Valley just across the border from Iraq, the Syrian Observatory for Human Rights said.

"Ten Iraqis from pro-Iranian militias were killed," the Britain-based monitor said.

The strikes came as tensions mounted between arch-foes Iran and the United States after Washington blamed Tehran for weekend attacks on Saudi oil installations.

They were the second to hit pro-Iranian forces in eastern Syria in little more than a week.

On September 9, air strikes killed 18 fighters, including Iranians, according to the Observatory.

Those strikes were blamed on Israel by a media outlet run by Iran's Lebanese ally Hezbollah.

Neither Israel nor the Syrian government made any comment.

In June last year, strikes near the Iraqi border killed 55 fighters, most of them Syrian or Iraqi. A US official speaking on condition of anonymity said Israel was responsible.

Much of the east of Syria was held by jihadists of the Islamic State group before their defeat in March.

It is now divided by the Euphrates Valley into a zone held by forces loyal to the Syrian government and its ally Iran and another held by the Kurdish-led Syrian Democratic Forces and their allies in a US-led coalition, which has in the past carried out air raids on pro-regime forces.