Syria's leader on Sunday vowed accountability and an investigation after the killing of Alawite civilians triggered an international backlash against the worst violence since Bashar al-Assad's overthrow.

In its latest toll, the Syrian Observatory for Human Rights war monitor said 830 Alawite civilians were killed in "executions" carried out by security personnel or pro-government fighters in the coastal provinces of Latakia and Tartus.

The Mediterranean area is the heartland of the Alawite minority community to which Assad, the toppled president, belongs.

United Nations rights chief Volker Turk said the killings "must cease immediately," while the Arab League, the United Nations, the United States, Britain and other governments have condemned the violence.

"We will hold accountable, firmly and without leniency, anyone who was involved in the bloodshed of civilians... or who overstepped the powers of the state," Syria's interim president Ahmed al-Sharaa said in a video posted by state news agency SANA.

Earlier Sunday, the presidency announced on Telegram that an "independent committee" had been formed to "investigate the violations against civilians and identify those responsible for them," who would face the courts.

Fighting between the new security forces and loyalists of the former government erupted on Thursday, after earlier tensions, and escalated into reported mass killings.

The fighting has killed 231 members of the security forces and 250 pro-Assad fighters, according to the Observatory, taking the overall death toll to 1,311.

Sharaa, in a separate address from a Damascus mosque, appealed for national unity.

"God willing, we will be able to live together in this country," he said.

'They gathered all the men'

Images on social media showed Syrian security forces on pickups and trucks driving past thick black smoke that drifted over the road on their way into the city of Jableh, between Latakia and Tartus.

The interior ministry said on Sunday that government forces were conducting "sweeping operations" in an area of Tartus province to "pursue the remnants of the toppled regime."

SANA quoted a defence ministry source as saying there were clashes in Tanita village in the same area.

An AFP photographer in Latakia city reported a military convoy entering a neighbourhood to search homes.

In Baniyas, a city further south, resident Samir Haidar, 67, told AFP two of his brothers and his nephew were killed by armed groups that entered people's homes, adding there were "foreigners among them."

"They gathered all the men on the roof and opened fire on them," Haidar said.

The mass killings followed clashes sparked by the arrest of a wanted suspect in a predominantly Alawite village, the Observatory said, reporting a "relative return to calm" in the coastal region on Saturday.

Chief US diplomat Marco Rubio said Syria "must hold the perpetrators of these massacres against Syria's minority communities accountable," while Britain's Foreign Secretary David Lammy said Damascus authorities "must ensure the protection of all Syrians and set out a clear path to transitional justice."

In Jordan, Syria's Foreign Minister Asaad al-Shaibani said that, "Anyone involved in this matter will be referred to the judiciary."

The semi-autonomous Kurdish administration in Syria's north and east condemned the "crimes" and underlined "that these practices take us back to a dark period that the Syrian people do not want to relive."

'Rule of law'

Sharaa's Islamist group Hayat Tahrir al-Sham (HTS), which led the toppling of Assad in December, has its roots in the Syrian branch of Al-Qaeda. It is still listed as a terrorist organisation by the United States and other governments.

The group has sought to moderate its image in recent years. Since the rebel victory, it has vowed to protect Syria's religious and ethnic minorities.

The new government has received diplomats from the West and its neighbours. It is seeking an easing of sanctions along with investment to rebuild a country devastated by 13 years of civil war under the repressive rule of Assad.

Sharaa has said Syria must be built "on the rule of law."

Israeli Foreign Minister Gideon Saar, speaking to German newspaper Bild, said Europe "must wake up" and "stop granting legitimacy" to the new Syrian authorities who he insisted were still jihadists.

The Alawite heartland has been gripped by fear of reprisals for the Assad family's five-decade rule which included widespread torture and disappearances.

Social media users have shared posts documenting the killing of Alawite friends and relatives.

The Britain-based Observatory, which relies on a network of sources in Syria, reported multiple "massacres" in recent days, with women and children among the dead.

During a sermon in Damascus, the Greek Orthodox Patriarch of Antioch John X said Christians were among those killed and called on Sharaa to "put a stop to these massacres... and give a sense of safety and security to all the people of Syria, regardless of their sect."

Later on Sunday, Syrian security forces fired into the air to disperse rival protesters in Damascus who engaged in physical altercations over the killings in the coastal areas.