Obama said in February he was deploying an extra 17,000 troops to Afghanistan to take on a resurgent Taliban.

As part of the review on US policy on Afghanistan and Pakistan unveiled in March, he also said he was sending 4,000 US troops to train Afghan security forces.

Obama's strategy also included proposals to counter persistent attacks by Taliban and al-Qaeda fighters across the border with Pakistan and additional development aid for both nations.

The Pakistani government has also made a number of deals with fighters there in an attempt to head-off their growing strength.

The US currently has about 38,000 troops in Afghanistan in addition to around 42,000 Nato troops.

The Taliban-led campaign has steadily gained strength in recent years, with 2008 the deadliest year so far.

Armitage left the Bush administration after he was re-elected president in November 2004.

He announced his resignation the day after his superior, Colin Powell, Bush's secretary of state, quit.