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, according to a report by a war monitoring group.

After hundreds of civilians were killed in a few days in areas dominated by the country’s Alawite religious minority, U.S. Secretary of State Marco Rubio stated that the U.S. would “watch the decisions made by the interim authorities.” He also expressed concern about “the recent deadly violence against minorities.”

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

According to the Syrian Network for Human Rights, a civil war monitoring group, the violence in recent days “included extrajudicial killings, field executions, and systematic mass killings motivated by revenge and sectarianism.”

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

The violence began when pro-Assad militants ambushed security forces last Thursday and killed more than a dozen. Following this, the government increased security forces in the coastal region.

The Syrian Network for Human Rights stated that armed groups and foreign Islamist fighters aligned with the government but not yet integrated into it were primarily responsible for the sectarian and revenge-driven mass killings. Thousands of foreign fighters came to Syria amid the civil war, many joining Islamist rebel groups opposed to the Assad regime.

The Syrian government did not immediately respond to a request for comment on the report.

Another war monitoring group, the Syrian Observatory for Human Rights, stated that more than 1,300 people have been killed since Thursday, adding on Tuesday that more than 1,000 civilians were among the dead. Most of the killings took place in Latakia and Tartus Provinces.

The Syrian Network for Human Rights has not provided tolls for the overall number of dead since the violence began. However, the group stated that more than 800 people were killed from Thursday to Monday — both civilians and combatants — in “extrajudicial killings.”

According to the report, “non-state armed groups” loyal to Assad were responsible for nearly half of those deaths, including almost 200 members of state security forces. The group also stated that it had not documented deaths of those non-state armed group members themselves during clashes.

The group also mentioned in its report that the large number of groups involved in the conflict and the confusion over their exact roles during this transitional period makes it “extremely difficult to determine individual legal responsibility” for the violence.

The figures provided by both war monitors could not be independently verified, and the reason for the discrepancies was not immediately clear. However, the situation has been murky, and exact numbers of civilians and fighters killed have been difficult to pin down during the chaos of recent days.

The U.N. Human Rights Office stated on Tuesday that it had documented the killing of 111 civilians so far, but it was still verifying the figures and the actual number is “believed to be significantly higher.”

The United Nations’ high commissioner for human rights, Volker Türk, urged the government on Tuesday to ensure that the investigations are “prompt, thorough, independent, and impartial.”

Syria’s new government has called for a complex web of armed groups across the fractured country to dissolve, and several prominent militias have agreed to work with the new authorities. However, the security situation has remained unstable, and it appears that all the militias have yet to be fully integrated into a single national army.

The interim Syrian president, Ahmed al-Shara, stated on Sunday that the government is forming a fact-finding committee to investigate the violence and bring the perpetrators to justice. Syrian officials have blamed Assad loyalists for the unrest and have not acknowledged any responsibility for the bloodshed.

A small number of gunmen have been arrested by government security forces in recent days after videos on social media showed civilians being killed.

Mr. al-Shara’s government is under intense pressure to bring stability to the country after more than a decade of civil war. But sectarian tensions are threatening to undermine his pledges to unite the nation and protect Syrians of all ethnic and religious backgrounds.