Damascus (AFP) - US-led coalition aircraft hit a Syrian army position in the country's east on Saturday, an armed forces statement said, while a monitor group reported dozens of soldiers dead.

"Warplanes from the American coalition hit one of the Syrian army's positions... near the Deir Ezzor airport," the statement carried by state media said.

It said the raid was carried out at 5:00pm local time (1400 GMT) and led to an unspecified number of "casualties".

Regime ally Russia said 62 Syrian soldiers were killed and at least 100 more wounded in strikes by "warplanes from the international anti-jihadist coalition".

The Syrian Observatory for Human Rights monitoring group said at least 83 soldiers were killed in the raids on Jabal Therdeh, southwest of the Deir Ezzor airport, updating an earlier toll of 30 killed.

Observatory head Rami Abdel Rahman said he could not specify who carried out the strikes.

Syria's army has been fighting off a fierce offensive by the Islamic State jihadist group on the Deir Ezzor military airport since last year.

"This is a dangerous and bold attack against the Syrian state and army, and clear evidence that the United States and its allies support the terrorist group Daesh," the statement said, using an Arabic acronym for IS.

It said the raids had allowed IS to advance on Syrian troops in the area.

The IS-linked Amaq news agency said coalition strikes hit IS positions on Saturday afternoon, but that the jihadist group was able to "seize full control of Jabal Therdeh, which overlooks the Deir Ezzor airport".

A Syrian military source inside the Deir Ezzor airport told AFP that US-led coalition strikes hit two hilltops near the airport.

"There were three raids on two army positions held by the Syrian army on Jabal Therdeh," the source said.

"Then, there was machine gun fire on the wounded soldiers and the paramedics" from the planes, he said.

"We were the most unsettled by the machine guns."

Under a fragile truce deal negotiated by the United States and Russia, fighting in Syria is to halt on all fronts across the country except those held by IS.

The deal came into force on Monday. If it holds for a week, Moscow and Washington are to begin unprecedented joint operations against jihadist groups in Syria.

The US-led coalition has been carrying out raids against IS for two years in both Syria and Iraq.