UN-Arab League envoy Lakhdar Brahimi speaks to the press on April 19, 2013, at the United Natuions in New York. (RICK BAJORNAS/AFP/GETTY IMAGES)

Lakhdar Brahimi, the U.N.-Arab League envoy for Syria, has informed senior U.N. diplomats that he intends to resign in the coming weeks, marking the end of another doomed U.N. diplomatic effort to end a bloody civil war that has left over 70,000 dead, according to U.N.-based diplomats.

The decision pitches the world’s main diplomatic initiative on Syria into a state of crisis at a time when the United States and its allies are weighing a response to new intelligence reports indicating that Syria may have used chemical weapons against its people. It comes as Ake Sellstrom, the U.N.’s newly appointed chemical weapons inspector, arrived in Washington for meetings on the Syrian program.

The United States has sought to persuade Brahimi to put off his plans to step down until after U.S. Secretary of State John F. Kerry concludes a May 7-8 visit to Moscow for meetings on Syria and other matters with Russian Foreign Minister Sergei Lavrov.

The State Department would not confirm the resignation and referred questions to Brahimi. Addressing persistent reports of Brahimi’s likely exit earlier this week, State Department spokesman Patrick Ventrell said the Obama administration hoped he would stay on. Kerry met with the envoy Monday in Washington in hopes of persuading him to continue.

Martin Nesirky, chief spokesman for U.N. Secretary General Ban Ki-moon declined to confirm Brahimi’s resignation plans. But a U.N.-based diplomat from a government that has been briefed on the matter by Brahimi did. He and other diplomats spoke on the condition of anonymity to discuss internal U.N. debate. “He said he’s going to resign,” the diplomat said. But he said Brahimi would delay a formal announcement to allow the United Nations time to “make arrangements for a transition.”

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The U.N. secretary general, meanwhile, has been in discussions with the United Nations’ five major powers — Britain, China, France, Russia and the United States — about the future of U.N. diplomatic efforts after Brahimi’s departure.

Brahimi, a veteran U.N. troubleshooter who has led major peace efforts from Afghanistan to Iraq, voiced increasing despair recently over the dwindling prospects for a political transition in Syria.He has faulted the Syrian government and the armed opposition for failing to recognize the futility of a military victory and the need for a negotiated settlement.

“I am personally, profoundly sorry that my own efforts have produced so little,” he told the Security Council in a closed-door meeting last month. “I apologize to the Syrian people for having, in the end, done so little for them during these past eight months and to you, in this council, for having had only sad news to report to you.”

One senior Western diplomat who met with Brahimi in recent weeks said that the U.N. envoy had expressed frustration with a March 6 decision by the Arab League to adopt a resolution authorizing the Syrian National Coalition, the main Syrian opposition group, to represent Syria at the Arab League. The resolution, he explained to the Security Council last month, constituted a recognition that “no dialogue or negotiations are possible or necessary.”

Anne Gearan in Washington contributed to this report.