DAMASCUS -- Syria accused the U.S. on Thursday of a "new barbaric aggression" after rare coalition airstrikes in an eastern province killed dozens, with Damascus calling it an effort to "support terrorism."
The Syrian Foreign Ministry urged the U.N. Security Council in a letter to condemn the "brutal massacre" in the eastern province of Deir al-Zour, state news agency SANA reported.
Overnight U.S.-led coalition strikes killed at least 45 Syrian government soldiers and their allies, according to the Britain-based Syrian Observatory for Human Rights, a war monitor.
The coalition said it launched a series of "defensive strikes" after pro-Syrian government forces "initiated an unprovoked attack" on a headquarters of the U.S.-allied Syrian Democratic Forces (SDF).
The U.S.-led alliance backs the SDF fighters, a grouping of mainly Kurdish and Arab militias, in their fight against Islamic State in eastern Syria.
The U.S. usually tries to avoid direct military confrontation with Syrian government forces and their allies.
Coalition partners had been with the SDF during the attack, which took place on Wednesday, five miles east of the Euphrates River de-confliction line, the coalition said in a statement.
"In defense of coalition and partner forces, the coalition conducted strikes against attacking forces to repel the act of aggression against partners," the statement continued.
But the Syrian Foreign Ministry letter said: "This new aggression, which represents a war crime and a crime against humanity and a direct support to terrorism, affirms the nature of the low U.S. intentions against the sovereignty of Syria."
It called on the U.N. Security Council to hold the international coalition responsible for killing and wounding thousands of civilians, according to Syrian state media.