Washington ? The White House is considering at least two troop withdrawal options as it weighs a new Iraq strategy — one that would preserve President Barack Obama’s campaign pledge to get all combat brigades out within 16 months and a second that would stretch it to 23 months, two officials said Friday.

A third, in-between option of 19 months is also being weighed, according to the officials, neither of whom would discuss the sensitive topic without being granted anonymity. One of the officials said the main focus appears to be on the 16-month and 23-month options; 23 months would run to the end of 2010.

Under either timeline, the U.S. would hope to leave behind a number of brigades that would be redesigned and reconfigured as multipurpose units to provide training and advising for Iraqi security forces, one official said. These brigades would be considered noncombat outfits and their presence would have to be agreed in advance by the Iraqi government, which under a deal signed late last year insisted that all U.S. forces — not just combat brigades — be out of Iraq by the end of 2011.

Obama has said his Iraq policy will include leaving a residual U.S. military force of unspecified composition and size in Iraq and in the region to conduct counter-terrorism missions against al-Qaida in Iraq and to protect American diplomatic and civilian personnel. He has said they will not build permanent bases in Iraq but will continue training and supporting Iraqi security forces “as long as Iraqi leaders move toward political reconciliation and away from sectarianism.”

The concept of the stay-behind training and advising brigades has been well developed, the official said, although the details such as their size and makeup are in an early stage of being sorted out.

At the White House’s request, top military officials recently offered an assessment of the risks associated with the 16-, 19- and 23-month withdrawal timetables, without saying which is preferred. Obama’s top two defense advisers, Defense Secretary Robert Gates and Joint Chiefs Chairman Adm. Mike Mullen, have not yet provided a formal recommendation to the president on a timetable, an official said.

It is possible that Obama will ask for similar assessments of other withdrawal timetables before deciding on a way ahead.

A senior administration official said Friday, without commenting on the timetables under consideration, that the White House and senior military commanders “are coming to a meeting of the minds” on troop withdrawals and on the need for a diplomatic and political strategy to end U.S. involvement in the war and to ease the strain on troops and their families.

The official, who spoke on condition of anonymity because he was not authorized to discuss the matter publicly, said it likely would be a matter of “weeks, not months” before Obama is ready to announce his decisions on an Iraq strategy.

McClatchy Newspapers was first to report Friday that the White House had received risk assessments associated with 16-, 19- and 23-month drawdown options.

Obama must weigh a number of risks in deciding how fast to pull out the 14 combat brigades that are now in Iraq, including the political risk associated with abandoning his campaign pledge to get out within 16 months.

The calculation is complex and tied to other concerns: relieving stress on war-weary troops and their families; tradeoffs in escalating the war in Afghanistan, and being ready for popup crises elsewhere.

The pace and sequencing of a troop pullout will have implications for preserving recent gains in reducing violence in Iraq. An erosion of security could in turn halt progress toward political reconciliation, raising once again the prospect of widespread sectarian warfare and a new crisis for Obama.