Kofi Annan has quit as peace envoy to Syria with a blistering attack on world powers, ending a frustrating six-month effort that failed to achieve even a temporary ceasefire as the country plunged into civil war.

Former United Nations secretary general Mr Annan also had harsh words for the Syrian regime, saying it was clear President Bashar Assad "must leave office".

Speaking in Geneva, Switzerland, Mr Annan blamed the Syrian government's intransigence, the growing militancy of the rebels and a divided UN Security Council that failed to forcefully back his effort.

Since he took on the job, Russia and China have twice used their veto power to block strong Western and Arab-backed action against Assad's regime.

The White House said Mr Annan's resignation highlighted the failure of Russia and China to support action against Assad and called the regime's continued violence against its own people "disgusting".

"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," said Mr Annan, a Nobel Peace Prize laureate. "You have to understand, as an envoy, I can't want peace more than the protagonists, more than the security council or the international community for that matter."

Mr Annan singled out the Syrian regime for blame over the violence. But he also said the opposition's increasing militarisation had contributed to dooming his six-point peace plan, which included a ceasefire and a Syrian-led political process to end the crisis.

"The bloodshed continues, most of all because of the Syrian government's intransigence, and continuing refusal to implement the six-point plan, and also because of the escalating military campaign of the opposition - all of which is compounded by the disunity of the international community," he said. "At a time when we need - when the Syrian people desperately need action - there continues to be finger-pointing and name-calling in the security council."

UN secretary general Ban Ki-moon said he accepted the resignation with deep regret, adding that the search was under way for a successor to Mr Annan, who will stay on until August 31. Diplomacy could succeed only when "the parties to the violence make a firm commitment to dialogue, and when the international community is strongly united in support," Mr Ban said.

In an opinion piece published by the Financial Times on Thursday, headlined "My Departing Advice on How to Save Syria", Mr Annan had harsh words for all parties in the conflict. But he appeared to reserve particular criticism for the Assad regime, asserting in his strongest statement to date about the Syrian leader: "It is clear that President Bashar al-Assad must leave office."

The Press Association, photo by World Economic Forum