The head of the departing United Nation's observer mission to Syria has acknowledged "some frustrations" at the failure to reduce violence and end the Syrian government's use of heavy weapons against its people.

The last 100 of the 300 monitors were set to leave the country on Sunday.

The UN observers' departure comes after the Security Council agreed to end the mission and support a small new liaison office that will support any future peace efforts.

Experts say the end of the observers' mandate represents a serious blow to international efforts to end the conflict.

Al Jazeera's Scott Heidler looks at what the UN mission achieved and what future role the organisation can play.