The Obama administration’s special Mideast envoy, former Sen. George Mitchell, is stepping down after more than two years of trying to restart peace talks between Israel and the Palestinians, a U.S. official with knowledge of the situation told NPR.

The White House was expected to release a statement from President Obama later Friday on the resignation of the veteran mediator and broker of the Northern Ireland peace accord.

There are no imminent plans to announce a replacement for Mitchell, although his staff is expected to remain in place at least temporarily, U.S. officials told The Associated Press, which first reported the news of Mitchell’s resignation.

The change comes at a critical time for the Middle East, which is embroiled in turmoil, and the Israel-Palestinian peace process, which has been moribund since last September and is now further complicated by an agreement between Palestinian factions to share power. Obama is expected to deliver a speech next week about his administration’s views of developments in the region ahead of a visit to Washington, D.C., by Israeli Prime Minister Benjamin Netanyahu.

Since his appointment on Obama’s second full day in office in January 2009, Mitchell, 77, has spent much of his time shuttling among Israel, the Palestinians and friendly Arab states in a bid to restart long-stalled peace talks that would create an independent Palestinian state. But in recent months, particularly after the upheaval in Arab countries that ousted longtime U.S. ally and key peace partner Hosni Mubarak from power in Egypt, his activity has slowed markedly.

NPR’s Michele Kelemen and Scott Horsley reported from Washington, D.C., for this story, which contains material from The Associated Press. Copyright 2011 National Public Radio. To see more, visit http://www.npr.org/.