Armed groups and foreign fighters linked to the government but not yet integrated into it were primarily responsible for sectarian massacres in Syria’s coastal region over the past week, a war monitoring group said in a new report.

The U.S. secretary of state, Marco Rubio, said on Wednesday that the U.S. would “watch the decisions made by the interim authorities” after hundreds of civilians were killed in just several days in areas dominated by the country’s Alawite religious minority. He added that Washington was concerned by “the recent deadly violence against minorities.”

The ousted dictator Bashar al-Assad was an Alawite and some members of his minority community enjoyed a privileged status under his rule.

The Syrian Network for Human Rights, which monitors the country’s civil war, said in a report released late on Tuesday that the violence in recent days “included extrajudicial killings, field executions, and systematic mass killings motivated by revenge and sectarianism.”

The clashes erupted almost a week ago in Latakia and Tartus Provinces — the Alawite heartland of Syria — between fighters aligned with the new government and Assad loyalists. The new government is led by Islamist former rebels who fought Mr. al-Assad in a 13-year civil war.

The violence was triggered when pro-Assad militants ambushed security forces last Thursday and killed more than a dozen of them. The government then poured security forces into the coastal region.