BEIRUT (AP) - Kofi Annan announced his resignation Thursday as Syria's peace envoy and issued a blistering critique of world powers, bringing to a dramatic end a frustrating six-month effort that failed to achieve even a temporary cease-fire as the country plunged into civil war.
In a reflection of that escalation, rebels used a captured tank to shell a military air base near Aleppo - one of the first known uses of heavy weapons by the insurgents.
Speaking to reporters in Geneva, Annan blamed the Syrian government's intransigence, the growing militancy of Syrian rebels and a divided 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 President Bashar Assad's regime.
The White House said Annan's resignation highlights 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 Annan, a Nobel Peace Prize laureate and former U.N. secretary general.
"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."
Annan singled out the regime for blame for the violence. But he also said the opposition's increasing militarization had contributed to dooming his six-point peace plan, which included a cease-fire 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.
U.N. Secretary-General Ban Ki-moon said he accepted the resignation with deep regret, adding that the search was under way for a successor to Annan, who will stay on until Aug. 31. Diplomacy can succeed only when "the parties to the violence make a firm commitment to dialogue, and when the international community is strongly united in support," Ban said in a statement.
Annan had managed, at least in theory, to get world powers behind his plan - including the Syrian government and its allies in Iran, Russia and China - although his appeals for peace were ignored on the ground.
The regime and the rebels blamed each other for the violence, and Russia and China said attempts to sanction the regime ignored violations by the opposition.
Asked about Annan's resignation, the White House again called for Assad to step aside.
The Syrian leader "continues to brutally murder his own people, to use heavy weapons in assaults on civilian population centers, to call on his military leaders to kill the Syrian people in his name," White House press secretary Jay Carney said. "It is disgusting and only highlights the absolute requirement that, for the future of the Syrian people, Assad must step aside."
Russia's U.N. envoy Vitaly Churkin said Moscow regrets Annan's decision and Syria's Foreign Ministry also expressed "dismay."