At least 90 regime soldiers were killed in a U.S.-led coalition air strike at an army position near Deir el-Zour airport on Saturday, the Syrian Observatory for Human Rights, a Britain-based monitoring group said, citing a source at the airport.

The Syrian Observatory for Human Rights added that the raid, which the U.S. said was unintentional, lasted 40 minutes, hitting several Syrian positions near an airbase in the eastern province of Deir al-Zour.

The Observatory initially estimated that 83 Syrian soldiers were killed and 120 injured in the Saturday attack that sparked a row between Russia, a key ally of Assad, and the US.

US Defense Department spokesman Peter Cook said the pilots believed they were striking Daesh targets, and that Russia had been notified earlier in the day at its plans to operate in the area but that "no concerns were voice."

Australia said Sunday that its planes were among the coalition aircraft over Deir el-Zour. "While Syria remains a dynamic and complex operating environment, Australia would never target a known Syrian military unit," the Defense Department in Canberra said.

Russian jets had been bombing in the same area at the same time, the Observatory added. The source at the airport said the air strike had paved the way for Daesh fighters to overun the position at Jebel Tharda.

The U.S. military said it halted an air raid against Daesh in eastern Syria after being told by Russia that it might have struck Assad regime forces.

A U.S. military official said he was "pretty sure" the targets hit in the halted air strike were regime forces, and that the attack was carried out using U.S. intelligence after days spent tracking potential targets.

Speaking on condition of anonymity, the official said the incident had taken place at around 11 a.m. EDT (1500 GMT) and lasted about half an hour.

The U.S. Central Command issued a statement late Saturday, after the strike in Deir el-Zour.

The statement said "the airstrike was halted immediately when coalition officials were informed by Russian officials that it was possible the personnel and vehicles targeted were part of the Syrian military."

"Coalition forces believed they were striking a Daesh fighting position that they had been tracking for a significant amount of time before the strike," the statement said.

Central Command said Syria is a complex situation, with a variety of military forces and militias in close proximity, "Coalition forces would not intentionally strike a known Syrian military unit," it added.

The coalition will review the strike and the circumstances surrounding it to see if any lessons can be learned, said the Central Command.

It is not uncommon for the Coalition Air Operations Center to confer with Russian officials as a professional courtesy and to avoid risking conflict between U.S. and Russian aircraft, the statement concluded.

The U.S. is not known to have directly struck Syria's Assad regime forces at any point during the five-year conflict.

In return, Russia's foreign ministry said that U.S. air strikes on the Assad regime could jeopardize the Russia and the U.S.- brokered cease-fire in Syria that took effect on Monday.

The truce has largely held despite dozens of alleged violations.

"We have reached the terrifying conclusion that the U.S. is conniving with Daesh," said the Russian foreign ministry.

Maj. Gen. Igor Konashenkov said the airstrike took place in a military base near the airport in eastern Syria and was carried out by two F-16s and two A-10s. He did not identify the planes' country affiliation, but said they were part of the international coalition.

Konashenkov says regime sources told the Russians that 62 soldiers were killed and more than 100 wounded. He says the planes came from the direction of the border with Iraq.

Russia has bitterly complained about what it says is a U.S. reluctance to abide by the terms of a shaky ceasefire brokered by Moscow and Washington which it says made provision for joint targeting of Daesh and other militant groups.

"If this air strike was the result of a targeting error, it is a direct consequence of the U.S. side's stubborn unwillingness to coordinate its action against terrorist groups on Syrian territory with Russia," the defence ministry said in a statement.

It said that Daesh had immediately gone on the offensive after the strikes and that a fierce battle between them and the regime army was now under way in the area, where it said local residents had previously been receiving humanitarian aid by parachute over an extended period.

The ministry said the strikes had been carried out between 1700 and 1750 local time.

Russia is a key ally of Bashar Assad and has been carrying out airstrikes on behalf of his forces since last year.