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. He also said President Bashar al-Assad of Syria “must leave office.”

In an announcement tinged with bitterness and regret, Mr. Annan tied his decision to 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.

“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 said at a news conference at the Geneva offices of the United Nations.

But, he said, “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.”