WASHINGTON, Feb. 8 (Xinhua) -- U.S. Defense Secretary James Mattis said Thursday that the United States is not "getting engaged in the Syrian civil war," after deadly U.S.-led coalition airstrikes killed about 100 pro-Syrian government forces.

"It was self-defense. Obviously, we are not getting engaged in the Syrian civil war," Mattis told reporters at a briefing in Pentagon.

The U.S.-backed Syrian Democratic Forces (SDF) said earlier on Thursday that the U.S.-led coalition struck pro-Syrian government forces in the eastern Syrian province of Deir al-Zour on Wednesday to defend its partners.

The coalition's move was in retaliation for the pro-Syrian government forces' targeting of the U.S.-backed forces in the area, said the SDF.

Calling the attack a "perplexing situation," the Pentagon chief said that he could not give "any explanation for why" they would shell the SDF headquarters with artillery.

The Syrian Foreign Ministry on Thursday strongly condemned the U.S.-led airstrikes, accusing the U.S. warplanes of "committing a massacre" against pro-government forces.

The pro-government Syrian fighters were confronting IS militants, "who are under the protection" of the U.S. coalition and the allied forces of the SDF, it added.

The Syrian government urged the UN Security Council to condemn the massacre and to hold the U.S. coalition responsible for their "crimes and aggression that have left thousands of civilians either killed or wounded," said the ministry.