WASHINGTON - The Marine general tapped to take over as the top U.S. commander in Afghanistan told Congress on Tuesday that he supports U.S. President Barack Obama's decision to withdraw U.S. reinforcements by next September.

Under questioning by members of the Senate Armed Services Committee, Lt. Gen. John R. Allen said he had no say in the internal deliberations that led to the decision announced by Obama last week to withdraw 10,000 U.S. troops by the end of this year and as many as 23,000 more by September 2012.

"Although I was not a participant in those discussions," Allen said, "I support the president's decision and believe that we can accomplish our objectives."

Allen said the withdrawal will impress on Afghan leaders that they must urgently increase the number and capabilities of their own security forces to take over as U.S. troops leave. All foreign combat forces are to be gone by the end of 2014.

A leading critic of Obama's decision, Sen. John McCain, the top Republican on the committee, said the withdrawal schedule poses unnecessary risks to the security gains that have been made over the past year.

"At the moment when our troops could finish our main objective and begin ending our combat operations in a responsible way, the president has now decided to deny them the forces that our commanders believe they need to accomplish their objective. I hope I am wrong," McCain said.

He asked Allen whether Obama's decision will make his job harder or easier. Allen said he could not give a meaningful answer because he has not yet been confirmed by the Senate, but he noted that the schedule set by the president is "a bit more aggressive than we had anticipated."

In written responses to the committee prior to his hearing, Allen warned that success in Afghanistan is threatened by a significant lack of military trainers and mentoring teams for the Afghan Army and police.

Allen's hearing comes less than a week after Obama laid out his plan to withdraw 10,000 troops by the end of this year, and another 20,000-plus by the end of next September.

Gen. David Petraeus, the man Allen will replace if confirmed by the Senate, told Congress last week that he had recommended a more gradual withdrawal. Adm. Mike Mullen, chairman of the Joint Chiefs of Staff, said Obama's plan was more aggressive than he considered prudent.

Allen voiced none of that criticism in committee documents obtained by The Associated Press.

"I believe this reality sends an important message of commitment to the Afghan people, as well as a sense of urgency that the Afghans must take on more responsibility for securing their own country," he said in the questionnaire.

He repeated military assertions that the insurgents' momentum has been stopped in most of the country, and reversed in many key areas. And Allen also predicted more tough fighting ahead.

At the same time, the Marine officer acknowledged a number of critical challenges the U.S. faces in Afghanistan, including the lack of trainers. He said there is currently a shortfall of nearly 500 trainers, and a need for more than 200 mentoring teams for both the Army and the police.

As the Afghan security forces continue to expand, he said, the shortfalls will be more difficult to meet. And Allen said that filling the gap would be critical to the success of the overall operation.

Allen, who would get promoted to four-star general to take the command job, has been serving as the deputy commander at U.S. Central Command and earlier this month moved to become a special assistant to Mullen.

But he is best known for his role in the stewardship of the Anbar Awakening -- the ultimately successful campaign by Marines in western Iraq to encourage Sunni tribesman to turn against al Qaeda and align with American forces. Allen served as the deputy commanding general of Marine forces in the west from 20-2008.