BEIRUT (Reuters) - Syria’s government said a U.S.-led military coalition carried out a deadly air strike on a Syrian army camp, but coalition officials said the report was false.

Syria said 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 U.S.-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 U.S. military official, speaking on condition of anonymity, said the United States is certain that Russia was responsible for the deadly strike on the Syrian army camp.

The official flatly dismissed claims that U.S.-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 U.S. military official said indications pointed to a strike carried out by a Russian TU-22 bomber.

Brett McGurk, U.S. 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 U.S.-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 kilometers (35 miles) 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 U.S. defense 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 U.S. defense official said Deir al-Zor was among the locations Russia had targeted on Sunday. Russian officials were not immediately available for comment.

The U.S.-led coalition first launched air strikes against Islamic State in Syria in September 2014, after beginning aerial operations against the group in neighboring 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.

The province links Islamic State’s de facto capital in Raqqa with territory controlled by the group in Iraq.

‘FLAGRANT AGGRESSION’

Britain joined the United States and its allies last week in the bombing campaign against Islamic State in Syria, ahead of proposed international peace talks later this month. Syria’s fragmented opposition is set to meet in Riyadh this week in an effort to unify ahead of the talks.

Syria’s Foreign Ministry said jets fired nine missiles at the camp, state TV reported.

The ministry sent letters to the U.N. Secretary General and the head of the U.N. Security Council condemning the “flagrant aggression,” state news agency SANA said.

It urged the United Nations to take “immediate action and take the necessary measures to prevent a repeat” of the incident, it said.

The ministry said three armored vehicles, four military cars, a weapons cache and ammunition had also been destroyed.

The strikes “confirm once again that the American coalition lacks the seriousness and trust (needed) to fight terrorism in an effective way,” it said.

The Syrian government has only a limited presence in Deir al-Zor province, which is mainly controlled by Islamic State.

Russia’s air strikes have hit some Islamic State targets, but the United States and its allies say most of them have hit other foreign-backed rebel groups.

In Deir al-Zor city, another air strike overnight killed a woman and two of her children, the Observatory said.