'They left nobody': More than 600 people killed in some of Syria's deadliest violence

The death toll, reported by the Syrian Observatory for Human Rights (SOHR), makes the two days of violence some of the deadliest since Syria's conflict began 14 years ago, according to the group.

The government said it is responding to attacks from remnants of Assad's forces and blamed "individual actions" for the violence.

Four hundred and twenty-eight civilians were killed, as well as 120 pro-Assad fighters and 89 people from security forces, according to the British-based SOHR.

No official figures have been released.

SOHR said the violence erupted on Thursday afternoon when pro-Assad militants ambushed Syrian security forces in the Latakia region, a former stronghold of the deposed leader where many of the minority Alawites, who made up his support base, live.

On Friday, Sunni Muslim gunmen loyal to the government began revenge killings against members of Assad's minority Alawite sect, according to people in the region.

"They forcibly brought people down to the streets, then they lined them up and started shooting them," said one resident of Baniyasin on the west coast of Syria.

"They left nobody. They left nobody at all. The scene that I saw was pure horror; it's just indescribable," he said.

The man, who Sky News is not naming for his safety, described women being forced to "walk naked" before being shot and a teenage boy being handed a rifle and forced to shoot his family.

He said he hid in a bin before escaping under the cover of night.

"The victims of this massacre were all civilians - just civilians and not Assad loyalists," said the man.

Other residents from Alawite communities told the Associated Press (AP) about gunmen shooting Alawites, the majority of them men, in the streets or at the gates of their homes.

Many homes were looted and then set on fire, two residents of Syria's coastal region told the AP from their hideouts.

The revenge killings stopped early Saturday, according to SOHR's founder Rami Abdurrahman.

"This was one of the biggest massacres during the Syrian conflict," Mr Abdurrahman said about the killings of Alawite civilians.

Syria's state news agency said all roads leading to the coastal region have been closed "to prevent violations and gradually restore stability".

An unnamed Defense Ministry official was quoted by the news agency as saying government forces have regained control of much of the areas from Assad loyalists.

The bodies of 31 people killed in revenge attacks, including nine children and four women, the day before in the village of Tuwaym were laid to rest in a mass grave, residents said.

A funeral was held for four Syrian security force members in the northwestern village of Al-Janoudiya.

-SKY NEWS