The United Nation's troubled observer mission to Syria has officially ended after being recalled amid escalating violence as world powers fail to agree how to end months of bloodshed in the country.

The mandate of the UN Supervision Mission in Syria (UNSMIS) expired at midnight Sunday after a roughly four-month deployment in which its work was hobbled by growing unrest that has cost tens of thousands of lives.

Created after a UN Security Council resolution in April, a team of some 300 truce monitors was progressively deployed into Syria as part of UN-Arab League envoy Kofi Annan's six-point plan to end the conflict.

It was suspended in June and numbers cut back because of the mounting violence, as both sides violated a ceasefire that was meant to have been the cornerstone of Annan's plan.

The UN mission ended as President Bashar al-Assad made a rare public appearance for the Muslim holiday of Eid and activists staged protests across Syria to rage against the regime.

Assad joined prayers at a Damascus mosque for the Eid al-Fitr festival, his first appearance in a public place since a bomb blast last month killed four top security officials, although he has been seen on television since then.

But despite the religious holiday, his forces were still in deadly action on the ground, shelling several rebel hubs and clashing with opposition fighters in Damascus itself, a monitoring group said.

Six children, one as young as five and including four from the same extended family, were killed by shelling near their home in the rebel-held town of Maaret al-Numan in the northwestern province of Idlib, said the Syrian Observatory for Human Rights.

In all, at least 56 people - including 22 civilians - were killed on the first day of Eid, the festival celebrated by Muslims to mark the end of the holy month of Ramadan, the Britain-based group said.

The departing UN observer mission chief on Saturday accused both Syrian army and rebel forces of failing to protect civilians.

"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."

Annan, a former UN secretary general, steps down as international envoy on Syria at the end of this month after complaining about a lack of international support for his six-month campaign to make President Bashar al-Assad and opposition fighters end their hostilities.

The United Nations plans to maintain a political liaison office in Damascus to support the mediation efforts of his successor, veteran Algerian diplomat Lakhdar Brahimi.

Officials have said the liaison office would probably be made up of between 20 and 30 people, including political, humanitarian and military experts.

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

Syrians joined prayers and staged demonstrations for Eid, taking place for the second year under the shadow of a conflict the Observatory says has now claimed 23,000 lives while the UN gives a death toll of 17,000.

"Eid is here, Eid is here, God curse you, O Bashar," protesters in Qudsaya in Damascus province sang to the tune of Jingle Bells, according to amateur video posted on YouTube.

"There is no Eid for Bashar, nothing is holy for him. They are willing to strike anywhere, mosques, hospitals, bakeries, children. What kind of Eid is this?" Abu Issa, a 39-year-old builder in Aleppo, asked an AFP correspondent.

[image via Agence France-Presse]