"Both parties have obligations under international humanitarian law to make sure that civilians are protected," General Babacar Gaye, head of the UN Supervision Mission in Syria, told reporters in Damascus.

"These obligations have not been respected."

The general was speaking after the UN Security Council on Thursday ordered the end of the observer mission, whose mandate draws to a close at midnight on Sunday.

A team of some 300 truce monitors was progressively deployed as part of former UN-Arab League envoy Kofi Annan's six-point plan to end the conflict in Syria but it was suspended in June and numbers cut back because of the mounting violence.

"By the middle of June, it was clear that the parties were no longer committed to the ceasefire," Gaye said. "The result has been an escalation in violence."

But Gaye, a Senegalese general, said the UN remained committed to contributing to peace efforts in Syria. "The UN will not leave Syria," he said. "We will continue the search to continue from violence to dialogue.

UN chief Ban Ki-moon on Friday announced the appointment of veteran Algerian diplomat Lakhdar Brahimi to replace Annan as international envoy to Syria, even as world powers remain divided over how to end the conflict.

What began in March 2011 as a peaceful uprising demanding the fall of President Bashar al-Assad's regime has grown into a bloody insurgency, after the army and security forces launched a major crackdown across the country.

More than 23,000 people have been killed, according to the Britain-based Syrian Observatory for Human Rights, while the UN puts the death toll at 17,000 people.