Syria blamed the U.S.-led coalition for the deaths of three soldiers on Sunday night after warplanes attacked their camp. The Pentagon insists Russian forces, however, not Americans, conducted the airstrike. Russia is a staunch ally of Syrian President Bashar al-Assad.

“This hampers efforts to combat terrorism and proves once again that this coalition lacks seriousness and credibility to effectively fight terrorism,” wrote Syrian officials to the UN.

It is the first time Syria has accused the coalition of attacking their forces since the campaign began 14 months ago. Officials said the planes hit the post “with nine rockets late Sunday, killing three soldiers and injuring 13.” The bombs also destroyed military equipment.

“We did not strike any vehicles or personnel targets in this area. We have no indication any Syrian soldiers were even near our strikes,” stated the coalition.

A U.S. official told Fox News, “It is ‘highly likely’ Russia carried out” the attack, but did not provide more details. The Associated Press cited another official who said the Pentagon is “certain” Russia is responsible. From the AP:

U.S. officials cited coalition surveillance and other data they said showed that Russian aircraft launched airstrikes near the Syrian army camp. One senior official said the Russian aircraft were in the air around 9 p.m. Sunday near the city of Deir el-Zour, whereas the coalition aircraft were flying an hour earlier about 55 kilometers away.

A senior U.S. official said radar tracked a dozen Russian TU-22 bombers that left Mozdok, Russia, traveled over the Caspian Sea, across portions of Iran and Iraq, and launched airstrikes in Raqqa, Deir el-Zour and Al Bukamal, Syria.

Radar shows Russian bombers making seven passes over the city of Deir el-Zour dropping munitions, the official said, adding that one of the TU-22s made a pass over the area of the army camp within one minute of the explosion that killed the Syrian troops. Overall, the Russian bombers launched strikes in four locations around Deir el-Zour.

The Pentagon did not respond to Breitbart News’s request for comment at press time.

Spokesman Col. Steve Warren said the coalition conducted “overnight strikes in the province of Deir al-Zour on Sunday, but the targets were oils wells” located 34 miles away from the army camp.

Russian officials did not release a comment regarding the Pentagon’s findings. However, they expressed concern over the airstrike.

“Moscow is seriously concerned over reports that on Dec. 6 the U.S.-led coalition made an air strike on Syrian governmental positions in the Deir al-Zor area,” the Russian Defense Ministry wrote in a statement. “In general, these cases indicate that the situation in the areas of fighting with so-called Islamic State in Syria and Iraq is heating up.”

The U.S. coalition is also investigating claims their people killed 26 civilians on Monday night.

“Every time we get information about the possibility of a civilian casualty incident, we always do a credibility assessment on that information,” explained Colonel Steve Warren. “If the information is found to be credible, we’ll conduct an investigation, and we’ll release the results of that investigation. So we’re in that phase now, we’re assessing the credibility of the allegation, and if there is reason to believe that the allegation is credible, then we’ll conduct an investigation.”