The death toll from two days of clashes between Syrian security forces and loyalists of ousted President Bashar Assad and revenge killings that followed has risen to more than 1,000, a war monitoring group said Saturday, making it one of the deadliest acts of violence since Syria’s conflict began 14 years ago. The Britain-based Syrian Observatory for Human Rights said in addition to 745 civilians killed, mostly in shootings from close distance, 125 government security force members and 148 militants with armed groups affiliated with Assad were killed. It added that electricity and drinking water were cut off in large areas around the city of Latakia.

U.S. Secretary of State Marco Rubio condemned the killings in a statement Sunday. “The United States condemns the radical Islamist terrorists, including foreign jihadis, that murdered people in western Syria in recent days. The United States stands with Syria’s religious and ethnic minorities, including its Christian, Druze, Alawite, and Kurdish communities, and offers its condolences to the victims and their families,” Rubio said in the statement, adding, “Syria’s interim authorities must hold the perpetrators of these massacres against Syria’s minority communities accountable.”

The clashes, which erupted Thursday, marked a major escalation in the challenge to the new government in Damascus, three months after insurgents took authority after removing Assad from power. The government has said that they were responding to attacks from remnants of Assad’s forces and blamed “individual actions” for the rampant violence.

The revenge killings that started Friday by Sunni Muslim gunmen loyal to the government against members of Assad’s minority Alawite sect are a major blow to Hayat Tahrir al-Sham, the faction that led the overthrow of the former government. Alawites made up a large part of Assad’s support base for decades.