A series of air strikes killed three Syrian soldiers in the country's east, Syria's government said on Monday, accusing the US-led coalition of responsibility for the attack.
The US military denied any involvement.
If confirmed, it would be the first time coalition warplanes had hit Syrian government forces.
Air strikes hit the Saeqa military camp near the town of Ayyash in Deir ez-Zor province, according to the UK-based Syrian Observatory for Human Rights monitoring group. The group earlier reported that four soldiers were killed.
It was impossible to confirm which of the many air forces operating in the country was responsible for the attack.
The area is largely under the control of the Islamic State of Iraq and the Levant (ISIL), and the US-led coalition has regularly targeted the group in the eastern province.
Syria's air force is active in the region and its ally Russia also began launching strikes in the country in late September.
Syria's foreign ministry said on Monday that four fighter jets from the US-led coalition targeted a Syrian army camp with nine missiles - killing three soldiers and wounding 13 others.
The ministry said it sent letters to the United Nations denouncing the attack on its forces.
"The Syrian Arab Republic strongly condemns this flagrant aggression by the US-led coalition forces, which blatantly violates the objectives of the UN charter," said a ministry statement carried by state media.
The US military told Al Jazeera in an email that it conducted four strikes in Deir ez-Zor on Sunday - "all against oil well heads" - and 55km away from the town of Ayyash.
"We did not strike any vehicles or personnel targets in this area. We have no indication any Syrian soldiers were even near our strikes," the US military said. " We take all allegations of potential collateral damage seriously and will look into every allegation we receive."
A US military official, speaking on condition of anonymity, said Washington was "certain" that it was a Russian air strike that hit the camp, Reuters and AP news agencies reported.
There was no immediate comment from Russia's defence ministry.
Syria's foreign ministry announcement was the first to publicly declare an incident of this type since the US and allied countries began bombing ISIL more than a year ago.
The Syrian Observatory said the Syrian military camp was hit about 2km from an area controlled by ISIL.
In Deir ez-Zor city, another air strike overnight on Sunday killed a woman and two of her children, said the group, which relies on a network of activists, medical staff, and fighters on the ground for information.
US-led strikes 'kill dozens' of ISIL fighters in Syria
Also on Sunday, at least 32 ISIL fighters were killed in apparent US-led coalition raids, as Syria's President Bashar al-Assad slammed Britain's decision to join the fight.
The Syrian Observatory said the ISIL members were killed in about 15 strikes on the group's stronghold of Raqqa province in northern Syria.
The monitoring group's director Rami Abdel Rahman said air strikes hit an ISIL headquarters and bases to the north, east, and southeast of Raqqa city - its de facto capital.
Syria's conflict has taken the lives of more than 250,000 people, and another four million have been forced to flee the country since it erupted in March 2011.