Crocker, 58, who has spent the last 22 months in Iraq and worked for the Foreign Service for 37 years, wants to retire next January before a new administration arrives, The Washington Post reported Friday.

Iraq experts have cited concerns about Crocker's departure following so closely behind the departure of the top U.S. military commander, Army Gen. David H. Petraeus.

"A lot of people would concur that it's the best team we've had there. Why switch people out when you're having relative success? But then, there's not a good time," former diplomat Daniel P. Serwer said.

Since his March 2007 arrival in Iraq, Crocker headed an operation to prod the Shiite-led Iraqi government to use U.S. troops for enforcing reforms to help calm ethnic and religious tensions.