UN-Arab League envoy Kofi Annan told the UN Security Council on Tuesday his Syria peace plan could be the "last chance" to avoid civil war and that he fears torture and other rights violations are growing.

Annan said government forces "continue to press against the population" despite a cessation of hostilities that started on April 12, but attacks are more discrete because of the presence of UN monitors, according to diplomats at a closed briefing given by the envoy.

Annan briefed the 15-nation council on his efforts to get President Bashar al-Assad to implement his six-point peace plan which he said was possibly "the last chance to avoid civil war."

He stressed, however, that the peace bid was not an "open ended" opportunity for Assad, diplomats at the briefing said.

Annan said he had particular concerns that torture, mass arrests and other human rights violations are "intensifying."

People in Syria known for "advocating non-violence" in the 14-month-old uprising against Assad have been detained in recent days and week, he said.

The envoy said Assad bears "primary responsibility" for ending the military campaign and "creating an environment conducive to a political process."

The UN Security Council agreed to send 300 unarmed military observers to Syria to monitor the cessation of hostilities but the death toll has continued to pile up. The UN says more than 9,000 people have died in Syria.

The observer mission has an initial 90 day mandate running to mid-July but western nations have expressed doubts that it will be extended.