Senior U.S. defense officials say the White House is considering three different timelines for withdrawing U.S. troops from Iraq.

The officials, none of whom have been identified, told several Western news agencies that the Pentagon has submitted assessments of the risks associated with three options for troop withdrawal, in response to a White House request.

The shortest option is reported to be a 16-month pullout, in line with a pledge that President Barack Obama made during his campaign.  The White House also requested evaluations of a 19-month and a 23-month timeline, which officials say indicates Mr. Obama may retreat from his campaign promise to have U.S. troops out of Iraq by the middle of next year.

The risk assessments were first reported by the U.S.-based McClatchy Newspapers, and later confirmed by other news agencies - the Associated Press and the French news agency, AFP.

McClatchy quotes a senior administration official as saying Mr. Obama is likely to announce his strategy for Iraq by mid-March.

In January, Defense Secretary Robert Gates told reporters that the Pentagon's "obligation is to give the president a range of options and the risks associated with those options."  He said the 16-month plan was one of those options, but at the time did not say what the other ones were.

The White House said several days ago that Mr. Obama has spoken to Iraqi Prime Minister Nouri al-Maliki about the planning process for withdrawing U.S. troops from Iraq.  The White House says the U.S. president pledged to consult with the Iraqi government as the plans move forward.

Some defense officials and diplomats believe pulling the troops out too quickly could destabilize Iraq again and result in renewed violence.

The status-of-forces agreement the United States signed with Iraq late last year says U.S. troops must be out of the country by the end of 2011. 

Some information for this report was provided by AFP and AP.