The envoy, Geir Pedersen, was "alarmed" at "the potential for further escalation of an extremely fragile situation" after violent clashes around Damascus and Homs and Israeli strikes near the capital, his office said in a statement.

Israeli Prime Minister Benjamin Netanyahu said Israel had "carried out a warning action" and struck "an extremist group preparing to attack the Druze population" in Sahnaya, outside Damascus, where deadly sectarian clashes erupted overnight.

Syria has been rocked by violence between fighters from the Druze minority and armed groups linked to the government, a major challenge for the Islamist authorities who overthrew longtime ruler Bashar al-Assad in December.

Pedersen called for "immediate measures to ensure the protection of civilians, bring about calm, and prevent incitement of communal tensions" in the country.

He was also "alarmed" at reports of Israeli strikes, the statement said, adding: "These attacks must stop."