Top White House adviser on the Middle East Dennis Ross plans to step down next month, the Obama administration said Thursday, in what would be the second high-profile departure from the administration’s policy team for the region in six months.

Ross, who helped set the administration’s course on relations with Iran as well as the Israeli-Palestinian conflict, is returning to private life after a tenure marked by frustrations and setbacks as well as tumultuous change.

The White House accepted the resignation and hailed Ross’s “extraordinary record of public service.”

Ross “played a critical role in our efforts to apply unprecedented pressure on the Iranian government, support democratic transitions in the region and deepen our security relationship with Israel while pursuing Israeli-Palestinian peace,” press secretary Jay Carney said.

Current and former colleagues said Ross’s decision was not precipitated by policy disagreements, though some said his departure reflected the diminishing chances for a breakthrough in either Iran or the Arab-Israeli peace process through next year’s presidential elections.

“The odds for quick success on either of them is slim to none,” said Aaron David Miller, a former State Department adviser on Israeli-Palestinian negotiations who worked with Ross. Miller attributed the setbacks on Middle East peace to a combination of early mistakes by the president and “the reality of how hard this job is, and the size of the mountain that has to be climbed.”

Ross, in a brief statement released by the White House, said he was leaving his post with “mixed feelings.”

“I am grateful to President Obama for having given me the opportunity once again to work on a wide array of Middle Eastern issues and to support his efforts to promote peace in the region,” he said.

Ross, 62, has served as an adviser on Middle East affairs during four administrations and was the chief Middle East envoy for former President Bill Clinton. Weeks after Obama’s inauguration he was appointed special adviser to Secretary of State Hillary Rodham Clinton and then, in June 2009, moved to the White House to become a special assistant to the president on the Middle East, the Persian Gulf and South Asia. The administration’s special envoy for the Middle East peace process, former senator George Mitchell (D-Maine), quit his post in May.

Ross’s nearly three years with the administration included several notable setbacks and challenges, including stalled Arab-Israeli peace talks and a rejection by Iran of Obama’s early efforts to improve relations between the countries. Then, early this year, the Arab Spring rebellions swept away pro-U.S. governments in Egypt and Tunisia and triggered civil war in Libya and violent government crackdowns in Bahrain and Syria.

Ross was often accused by Palestinians of being pro-Israel, though his negotiating stances sometimes drew equally angry denunciations from hard-line Israelis. In recent months he repeatedly warned both groups that time was running out for a negotiated settlement that would avert bloodshed.

“The longer it takes to forge an agreement between Israelis and Palestinians, the harder it will be to forge a two-state solution that meets the needs of both sides,” he told a Jewish group in May.