Kofi Annan has submitted his resignation as special envoy to Syria, saying he plans to step down on August 31.
Secretary-General Ban Ki-moon said in a statement today he accepted the resignation with “deep regret.”
Mr Annan called a last-minute press conference in Geneva to speak to reporters this afternoon.
Mr Annan delivered blistering criticism of world powers’ failure to unite over the country’s escalating violence.
He said he accepted the role when it seemed the international community led by the UN Security Council could help end the violence, enforce a cease-fire and bring about a political transition.
But the former UN secretary-general told reporters he cannot go on when the 15-nation council provides no backing for his role, particularly because of the stand-off between its five veto-wielding members: Russia and China on one side, the United States, Britain and France on the other.
He was appointed special envoy in February of last year.
“When the Syrian people desperately need action, there continues to be finger pointing and name calling in the Security Council,” he told reporters.
“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 added.
Annan said the failed six-point plan commonly referred to as the Annan plan is, in fact, the Security Council’s plan.
He did not rule out the idea of a successor being appointed by the current UN secretary-general, Ban Ki-moon, since “the world is full of crazy people like me, so don’t be surprised if someone else decides to take it on.”
Ban said in a statement he is discussing possible successors with the Arab League.
Russia’s UN envoy Vitaly Churkin said that Moscow regrets the decision to step down, according to the RIA Novosti.
Churkin also said he was encouraged by Ban’s search for a replacement, however.
British Prime Minister David Cameron called on the United Nations to "ramp up" pressure on the regime of Syrian President Bashar Assad after Mr Annan made his announcement.
Mr Annan was appointed envoy of the UN and the Arab League in February and travelled repeatedly to Syrian capital Damascus in an effort to broker a ceasefire in the uprising, in which 19,000 have died.
His announcement today that he will not seek a renewal of his mandate when it expires on August 31 is a clear sign of acceptance that this approach has failed to deliver the hoped-for breakthrough.
The struggle between regime forces and rebels continued today in Damascus and Syria’s largest city Aleppo.
Syrian activists said rebels were using a tank captured from government troops to bombard a military air base in Aleppo. The Bashar regime said it had carried out a string of raids on rebels in neighbourhoods on the south of Damascus, killing and arresting a number of “terrorists”.
UN Secretary General Ban Ki-moon said in a statement that he and his Arab League counterpart Nabil Elaraby are in discussions on appointing a successor to Mr Annan.
Mr Ban said Mr Annan deserved “our profound admiration for the selfless way in which he has put his formidable skills and prestige to this most difficult and potentially thankless of assignments”.
And Russia’s ambassador to the UN Vitaly Churkin said: “We understand that it’s his decision. We regret that he chose to do so.
“We have supported very strongly Kofi Annan’s efforts. He has another month to go, and I hope this month is going to be used as effectively as possible under these very difficult circumstances.”
For Labour, shadow foreign secretary Douglas Alexander said: “Kofi Annan’s resignation is the latest sign of the frustration and despair felt by so many in the face of the ongoing failure of the international community to bring an end to the violence in Syria.
“He deserves our gratitude and respect for the way he took on such a difficult and harrowing task with such dignity and determination.
“This latest setback must not be an excuse for inaction, but instead demands a new diplomatic effort to end this conflict which has already gone on too long and cost too many innocent lives.
“If the international community is to live up its responsibility, it must come together now otherwise the conflict will continue long after the battle now raging in Aleppo has ended.”