Petraeus steps down after a mixed one-year stint in charge of the more than 140,000 international troops in the country. He was the architect of the counterinsurgency strategy that aimed to bring peace through emphasising decisive strikes against known insurgents and protecting the local population.

But as he leaves, it is unclear if either strategy has made Afghanistan any safer. Violent attacks have continued, though international military officials argue they are not as widespread or as intense as they would have been otherwise.

Allen, who officially took command at a ceremony in Kabul on Monday morning, 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 said however, that he does not expect the fight to be easy.

AP