The U.N. envoy for Syria voiced alarm Friday at reports of clashes and killings in coastal areas between Syrian caretaker authority forces and elements loyal to toppled president Bashar al-Assad's regime.

"While the situation remains fluid and we are still determining the precise facts, there is clearly an immediate need for restraint from all parties," Geir Pedersen said in a statement.

He also called for "full respect for the protection of civilians in accordance with international law".

His comment came after the fiercest attacks on the new authorities since Assad was ousted in December.

The Syrian Observatory for Human Rights (SOHR) said at least 185 people have been killed since the clashes erupted on Thursday along the country's western coast.

The Britain-based Observatory that the Syrian security forces "executed" 90 civilians on Friday in the Mediterranean heartland of ousted president Bashar al-Assad's Alawite minority, a war monitor said.

"Ninety Alawite civilians, at least six of them women, were executed by the security forces in the districts of Baniyas, Latakia and Jableh," SOHR added.

Assad is a member of the Alawite minority.

Pedersen said he was "deeply alarmed" by the reports emerging from the area, decrying in particular "very troubling reports of civilian casualties".

"All parties should refrain from actions that could further inflame tensions, escalate conflict, exacerbate the suffering of affected communities, destabilize Syria, and jeopardize a credible and inclusive political transition," he insisted.

Forces led by Islamist group Hayat Tahrir al-Sham launched the offensive that toppled Assad on Dec. 8, when he fled to Russia with his family.

Syria's new security forces have since carried out extensive campaigns seeking to root out Assad loyalists from his former bastions.

Residents and organizations have reported violations during those campaigns, including home seizures, field executions and kidnappings, which the authorities have described as "isolated incidents."