Syria's government says a US-led military coalition carried out a deadly air strike on a Syrian army camp, but coalition officials say the report is false.
Syria says four coalition jets killed three of its soldiers and wounded 13 in the eastern province of Deir al-Zor on Sunday evening, calling it an act of aggression, the first time Damascus has made such an accusation.
Any such strike by US-led coalition planes, which have focused their fire on Islamic State targets, would further complicate the increasingly regional conflict.
The Syrian Observatory for Human Rights monitoring group earlier reported that jets likely to be from the coalition hit part of the Saeqa military camp near the town of Ayyash in Deir al-Zor province, killing four Syrian army personnel.
But a US military official, speaking on condition of anonymity, says the US is certain that Russia was responsible for the deadly strike on the Syrian army camp.
The official flatly dismissed claims that US-led coalition jets were responsible.
Russia, a key ally of Syria, is waging its own air campaign in support of President Bashar al-Assad, and has also been striking in Deir al-Zor.
A second US military official said indications pointed to a strike carried out by a Russian TU-22 bomber.
Brett McGurk, US President Barack Obama's envoy to the coalition, also denied claims of coalition responsibility, saying on Twitter: "Reports of coalition involvement are false."
Colonel Steve Warren, a Baghdad-based spokesman for the US-led coalition, said the alliance had conducted four strikes in the Deir al-Zor province on Sunday, all against oil well heads.
"Our strikes were approximately 55 kilometres southeast of Ayyash. We did not strike any vehicles or personnel targets. We have no indication any Syrian soldiers were near our strikes," he said.
A US defence official, who declined to be named, dismissed the idea that the coalition would target the Syrian military.
"We are not at war with the Assad regime and have no reason to target the Syrian Army," the official said.
"We are aware that Russia conducted long-range bomber strikes into Syria (on Sunday)."
Another US defence official said Deir al-Zor was among the locations Russia had targeted on Sunday. Russian officials were not immediately available for comment.
The US-led coalition first launched air strikes against Islamic State in Syria in September 2014, after beginning aerial operations against the group in neighbouring Iraq the previous month.
Its strikes have regularly targeted Deir al-Zor province in eastern Syria, most of which is held by Islamic State, including oilfields that are a source of income for the militant group.
