U.N. Secretary-General Antonio Guterres expressed alarm Monday at the sudden escalation of violence in Syria's coastal region, where security forces have clashed since Thursday with fighters loyal to former President Bashar al-Assad, killing hundreds of civilians.

"The bloodshed in Syria must stop immediately, he said, and perpetrators of violations must be held to account," Guterres' spokesperson, Stephane Dujarric, told reporters. "The concerns of Syria's communities must be addressed in a meaningful manner."

Dujarric said the U.N. chief was alarmed by the scope of the violence that included "widespread summary killings, including of entire families, and the loss of one of our colleagues from UNRWA."

The staffer from the agency that assists Palestine refugees was caught in the crossfire last week on his way home from work during clashes between the cities of Homs and Latakia, UNRWA's commissioner-general said.

The fighters are members of the country's Alawite minority, the same religious group the Assad family belongs to. The Syrian authorities said their forces in the sect's coastal region near the port city of Latakia came under a calculated attack from Assad loyalists in an attempted insurrection that has now been put down. Scores of fighters on both sides were also reportedly killed in what was some of the worst fighting since the interim authorities seized power in mid-December.

The head of the caretaker government, Ahmed al-Sharaa, a former member of al-Qaida, announced Sunday there would be an independent investigation into all atrocities carried out against civilians and the security forces.

Earlier Monday, the U.N. Security Council met in a two-hour closed session requested jointly by the United States and Russia.

Diplomats said U.N. Syria envoy Geir Pedersen told them in a video briefing that tensions have been brewing for several weeks between Assad loyalists and the current authorities, erupting in sporadic clashes. But the recent violence, he said, appeared preplanned and coordinated, with fighters reportedly targeting military installations and public facilities such as hospitals.

Pedersen warned council members that the situation was dangerous and that violence could spread. He urged international support for a real political transition to prevent Syria from collapsing again.

Russian Ambassador Vassily Nebenzia told reporters after the meeting that the 15-nation council is discussing issuing a presidential statement on the situation. Council statements require consensus, and Nebenzia said everyone "was pretty unanimous" on the gravity of the situation.

U.N. humanitarian officials said thousands of Syrians have reportedly been displaced in coastal areas, and thousands have crossed into neighboring Lebanon. Six hospitals and several ambulances have been affected in the fighting, and many hospitals in the coastal area are in urgent need of medical supplies.

Syria has been mired in civil war for the past 14 years, after the Assad regime brutally put down peaceful protests during the Arab Spring. Last month, the U.N. Development Program said recovery could take at least a decade and estimated the war had cost the Syrian economy $800 billion in losses since 2011.