The head of the now-defunct UN observer team in Syria, General Babacar Gaye, left Damascus on Saturday after the Security Council called time on the troubled mission, his aides said.
The mission officially ended on at midnight last Sunday amid escalating violence and divisions at the UN Security Council over how to end a conflict now in its 18th month.
Created by a Security Council resolution in April, a team of some 300 truce monitors was progressively deployed into Syria as part of former UN-Arab League envoy Kofi Annan's six-point plan to end the bloodshed.
But the UN Supervision Mission in Syria 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.
Last week, Gaye accused both the Syrian army and rebel forces of failing to protect civilians as the fighting escalated.
"Both parties have obligations under international humanitarian law to make sure that civilians are protected," he told reporters in Damascus last Saturday. "These obligations have not been respected."
Annan himself, a former UN secretary general, formally steps down as international envoy on Syria at the end of this month after complaining about a lack of international support in the face of the mounting bloodshed.
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, who said Friday he was "scared" of the scale of the task ahead.
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 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.
At least 24,500 people have been killed, according to the Britain-based Syrian Observatory for Human Rights, while the UN puts the death toll at 17,000.