WASHINGTON (CNN) -- President Bush will announce his nominations for two top U.S. diplomatic posts Monday, the White House said.

Bush will nominate Zalmay Khalilzad, currently U.S. ambassador to Iraq, for the post of U.S. ambassador to the United Nations. And he will nominate Ryan Crocker, U.S. ambassador to Pakistan, to replace Khalilzad in Baghdad, a White House spokesman told reporters.

The selections were expected. Khalilzad, 55, a native of Afghanistan and a Sunni Muslim, has been the U.S. ambassador to Iraq since June 2005. He was previously U.S. ambassador to Afghanistan.

Crocker, 57, has previously served as U.S. ambassador in three countries bordering Iraq -- Lebanon, Kuwait and Syria. In 2003, he was also a high-ranking official in the Coalition Provision Authority, which governed Iraq after the U.S. invasion that toppled dictator Saddam Hussein.