The U.S. military commander nominated to take over in Afghanistan says President Barack Obama's plan to withdraw 33,000 troops from Afghanistan by September 2012 will not undermine the war effort there.

Lieutenant General John Allen told lawmakers Tuesday the troop withdrawal schedule is “more aggressive” than he and other military leaders had expected, but said he believed the U.S. would still be able to achieve its objectives.

Allen has been nominated to be the U.S. and NATO commander in Afghanistan, replacing General David Petraeus, who has been tapped to head the CIA. Allen was testifying before the Senate Armed Services Committee.

Some U.S. lawmakers and Afghans are concerned that gains made against the Taliban will be lost if foreign troops are withdrawn before Afghan forces are ready to take over security.

A number of Mr. Obama's top military advisers, including General Petraeus, and Admiral Mike Mullen, chairman of the U.S. Joint Chiefs of Staff, have said they initially advised a more gradual troop withdrawal. But ultimately they supported Mr. Obama's plan, which was announced in a televised address last week.

In his testimony Tuesday, Allen also discussed the importance for the Afghan war effort of the U.S. relationship with Pakistan. That relationship grew significantly tenser after the U.S. raid near Pakistan's capital that killed al-Qaida leader Osama bin Laden.

Vice Admiral William McRaven, who is Mr. Obama's nominee to take over the U.S. Special Operations Command, also testified Tuesday. McRaven, who oversaw the raid against bin Laden, said he did not expect Pakistan to take more action against certain militant groups that operate near the Afghanistan border. McRaven said this was because Pakistan lacks the capacity to increase its operations there, and perhaps also the willingness.