UN-Arab League joint special envoy to Syria Kofi Annan is leaving his post.
Mr Annan authored a six-point peace plan for Syria which was intended to bring an end to the fighting. But the plan was never fully adhered to by either side and the fighting has continued.
UN Secretary General Ban Ki-moon said Mr Annan would not renew his mandate when it expires at the end of August. He said he had accepted Mr Annan's resignation with "deep regret".
In Geneva, Mr Annan blamed both sides in the conflict, but said this was ''compounded by the disunity of the international community''.
''When the Syrian people desperately need action, there continues to be finger pointing and name calling in the Security Council,'' he said.
''It is impossible for me or anyone to compel the Syrian government, and also the opposition, to take the steps to bring about the political process.
''As an envoy, I can't want peace more than the protagonists, more than Security Council or the international community, for that matter," he said.
Russia and China have vetoed resolutions on the crisis three times, citing opposition to any action which might be seen as regime change imposed from outside.
Mr Annan said he did not rule out someone taking over the mediator's role from him, saying:
''The world is full of crazy people like me, so don't be surprised if someone else decides to take it on."
But he said a successor might choose another path, adding that the focus remained on political transition, saying President Bashar al-Assad "will have to leave sooner or later".
Russia's president Vladimir Putin says he regrets Kofi Annan is leaving his role as international peace envoy for Syria.
Mr Putin, who is a strong supporter of the Syrian president Bashar al-Assad, praised Mr Annan as a brilliant diplomat and a decent person.
Russian news agencies quoted Mr Putin as saying he hopes international efforts aimed at ending the violence in Syria continue.
China, which has consistently opposed international intervention in the conflict, expressed regret at Mr Annan's departure, as did Syria.
The White House blamed Russia and China for the resignation, saying it highlighted their failure at the UN to support meaningful resolutions that would have held Mr Assad accountable.