Six Republican senators urged US President Barack Obama on Wednesday to withdraw his nominee for ambassador to Baghdad, saying recent allegations of impropriety could shred his credibility.

Obama has nominated Brett McGurk as Washington's next envoy to Baghdad but the lawmakers said he "lacks the leadership and management experience necessary to head America's largest embassy."

They also insisted that racy emails that were leaked onto the Internet last week would undermine his ability to work well with Iraqis.

"There are strong concerns about Mr McGurk's qualifications, his ability to work with Iraqi officials, and now his judgment," wrote the senators, including Jim DeMint, Republican rising star Marco Rubio and James Inhofe.

The emails from 2008, some of a sexual nature, were apparently between McGurk and Wall Street Journal correspondent Gina Chon, who is now McGurk's wife. Her resignation from the newspaper was announced Tuesday.

"The public release of information detailing unprofessional conduct demonstrates poor judgment and will affect the nominee's credibility in the country where he has been nominated to serve," the senators wrote.

The White House backed McGurk on Wednesday, insisting that much of the opposition stemmed from lawmakers "who believe we should still be at war in Iraq."

McGurk reportedly helped work on negotiations with Iraqis in 2008 over a potential deal which would have left a small US military presence in Iraq.

But that deal fell through, and Obama ordered all US forces to withdraw from Iraq last year.

Obama spokesman Jay Carney did not comment about the letter and declined to address the allegations of email impropriety.

"I can tell you that the president put forward this nominee because he is qualified for the job and will serve ably when he's confirmed," Carney told reporters.

The senators have not put a formal hold on the nomination, and the Senate Foreign Relations Committee have scheduled a vote on McGurk's confirmation for June 19.