The scheduled withdrawal of U.S. troops from Iraq is on track, a Pentagon official said Wednesday, but the Obama administration has yet to decide how many troops might stay there on a revised mission to help train Iraqi forces.

"The drawdown has begun," Navy Capt. John Kirby, a Pentagon spokesman, told reporters. He referred to the departure from Iraq this week of about 700 members of a headquarters unit. Their departure marks the start of the withdrawal of the final 46,000 U.S. troops there.

Yet to be decided is the size and mission of any stay-behind contingent. The Iraqi government said last month that it is interested in negotiating the terms for a U.S. military group to continue training Iraqi forces beyond Dec. 31, when the last U.S. forces are to have departed under a 2008 agreement.

The administration is considering a number of options that could leave several thousand troops in Iraq to do training and possibly other missions.

If the Iraqis decide they don't need a reinforced U.S. training contingent, then only about 150 U.S. military members would remain in Iraq next year as part of the U.S. Embassy's Office of Security Cooperation. They would help train Iraqis on new military equipment like battle tanks.

James F. Jeffrey, the U.S. ambassador in Iraq, said Wednesday that a proposal to keep 3,000 troops in Iraq next year, as reported by some news organizations, has "no official status or credibility." Jeffrey said that proposal has not been a part of ongoing discussions in Baghdad where both governments have been weighing whether up to 10,000 US forces should stay beyond Dec. 31.

Army Gen. Martin Dempsey, who will take over as chairman of the Joint Chiefs of Staff at the end of the month, said Wednesday that he hasn't been part of the internal deliberations on the drawdown. At a ceremony, Dempsey was asked about reports that the U.S. might leave as few as 3,000 troops in Iraq.

"I haven't been exposed to the number," he said, adding that "we should all realize that the Iraqi government will also have a say in what size structure and what size force should remain and for what purposes."

On Capitol Hill, a senior State Department nominee said the administration has made no final decision on how many troops it may keep in Iraq. Wendy Sherman told her Senate confirmation hearing that the issue centers on Iraqi government interest in U.S. military trainers. Sherman is the nominee to become the undersecretary of state for policy.

Some Republicans in Congress are advocating a much larger U.S. military presence in Iraq beyond 2011. They argue that a smaller presence would unnecessarily risk an unraveling of the security gains that have been accomplished at the cost of thousands of American lives.

Sen. John McCain, the top Republican on the Armed Services Committee, said keeping as few as 3,000 troops in Iraq falls far short of what U.S. military commanders have told him is needed. McCain cited conversations he has had during numerous visits to Iraq over the years.

Among the concerns cited by U.S. commanders are Iraq's underdeveloped air defenses and its gaps in intelligence, surveillance and reconnaissance.

"It's in America's national security interest not to lose Iraq after the sacrifice of some 4,500 brave young Americans," McCain said on the Senate floor. "And the consequences of failure are obvious."

Pentagon press secretary George Little said the State Department is leading the troop discussions with Iraq. He declined to discuss the internal deliberations.


Associated Press writers Lara Jakes in Basra, Iraq, and Lolita C. Baldor in Washington contributed to this report.