Frustrated by the seemingly intractable Syrian conflict, Kofi Annan announced his resignation on Thursday as the special peace envoy of the United Nations and the Arab League, throwing new doubts on whether a diplomatic solution is possible.

In an announcement tinged with bitterness and regret, Mr. Annan said he could no longer do the job, blaming his decision on what he described as Syrian government intransigence, increasing militance by Syrian rebels and the failure of a divided Security Council to rally forcefully behind his efforts.

Ban Ki-moon, the United Nations secretary general, said in a separate announcement that the search was on for a successor to Mr. Annan, who will serve until the end of August, when his mandate expires. But there was no word on who might replace Mr. Annan.

“I accepted this task, which some called ‘mission impossible,’ for I believed it was a sacred duty to do whatever was in my power to help the Syrian people find a peaceful solution to this bloody conflict,” Mr. Annan told reporters at a hastily organized news conference at the United Nations’ Geneva offices.

But “without serious, purposeful and united international pressure, including from the powers of the region, it is impossible for me, or anyone, to compel the Syrian government in the first place, and also the opposition, to take the steps necessary to begin a political process,” he said.

Mr. Annan was especially critical of what he called the “disunity” of world powers. “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.”

It was no secret that Mr. Annan had grown increasingly frustrated over his failure to achieve even a basic cease-fire in the conflict, which began 17 months ago as a peaceful uprising against President Bashar al-Assad and has now escalated into civil war.

A Nobel Peace Prize winner and former United Nations secretary general, Mr. Annan, 74, is one of the world’s most seasoned diplomats. He agreed in February to act as a special representative for both the United Nations and the Arab League to negotiate a peace plan in the Syrian conflict, and received unanimous backing from the Security Council.

Within a few months he negotiated a six-point proposal that called for the Syrian government to withdraw its heavy weapons and troops from populated areas and for anti-Assad fighters to put down their guns. Other provisions included a process for a political transition that, in theory at least, would have replaced Mr. Assad, a member of Syria’s Alawite minority whose family has dominated Syrian politics for four decades.

Despite a pledge from Mr. Assad on March 27 to abide by the peace plan, the Syrian government never implemented it. Mr. Assad’s opponents, sensing that he had no intention of honoring his commitments, did not lay down their weapons either.

Although the Security Council supported Mr. Annan’s efforts, two of its permanent members with veto power, Russia and China, opposed any additional coercive measures that they feared could lead to outside military intervention in Syria.

The disagreement led to bitter recriminations on the council, pitting Russia and China against the United States, Britain and France, the three other permanent members, which had been pressing for a more forceful Syria resolution.

Mr. Ban noted in his statement about Mr. Annan’s resignation that the Security Council’s own divisions “have themselves become an obstacle to diplomacy, making the work of any mediator vastly more difficult.”

“Tragically, the spiral of violence in Syria is continuing,” Mr. Ban said in the statement. “The hand extended to turn away from violence in favor of dialogue and diplomacy — as spelled out in the six-point plan — has not been taken, even though it still remains the best hope for the people of Syria.”

Word of Mr. Annan’s resignation came as the United Nations General Assembly was preparing to vote on a resolution offered by Arab countries that demands that the Syrian government compliance with his plan. But the General Assembly resolution, which is scheduled for a vote on Friday, does not have the enforcement power of a Security Council measure, and has been viewed as largely a symbolic effort to embarrass Syria and its backers.