The head of the UN observer mission in Syria has said he was frustrated with the failure to reduce the violence and end the government's use of heavy weapons.

Lieutenant General Babacar Gaye was giving his final news conference in Damascus ahead of the end of the mission on Sunday.

The observers' departure follows the UN Security Council's decision to end the mission and replace it with a small liaison office.

The expiry of the observers' mandate represents a serious blow to international efforts to end the conflict.

Al Jazeera's Scott Heidler, reporting from the UN headquarters in New York, assesses what the mission achieved and what future role the UN can now play.