U.S. Gen. David Petraeus handed over command of international forces in Afghanistan to Marine Gen. John Allen on Monday.

Petraeus heads to his new job as director of the CIA, while Allen, known for helping turn Sunni insurgents against al-Qaeda in Iraq, takes over in Kabul. Allen will be tasked with the overseeing the start of the American troop withdrawal from the country after a decade-long war.

Allen, who was promoted to a fourth star as he took command of about 130,000 U.S. and NATO troops, has said he supports President Barack Obama's plan to withdraw a third of the U.S. forces in Afghanistan by next year.

But Allen told a Senate hearing last month that the schedule set by Obama was more aggressive than the military had anticipated. Obama did not set a minimum number of troops to be pulled out this month, but required only that 10,000 be gone by the end of the year and that another 23,000 be back home by September 2012.

Allen said the drawdown of U.S. forces that started earlier this month and the transition of some areas to Afghan control this week does not mean that international forces are easing up in their campaign to defeat the Taliban insurgency.

"It is my intention to maintain the momentum of the campaign," Allen said at the handover ceremony in the Afghan capital. He acknowledged, however, that the fight won't be easy.

"There will be tough days ahead. I have no illusions about the challenges," Allen said.

Before taking over from Petraeus, Allen had been serving as the deputy commander at U.S. Central Command in Tampa, Florida.   

There have been fears in Afghanistan that the U.S. decision to draw down its forces could lead to a precipitous withdrawal of other foreign troops. Foreign forces are expected to gradually hand over control of Afghanistan to government forces and end their combat role by the end of 2014.

Afghan Defence Minister Abdul Rahim Wardak said he hoped Petraeus' appointment to the role of CIA director will temper calls for a quick withdrawal of U.S. forces and funding.

Petraeus steps down at a time when violence has again spiked, with insurgents carrying out attacks against high-profile Afghans, including the assassination last week of President Hamid Karzai's powerful half brother and the slaying of a close Karzai aid on Sunday.