McChrystal presented a united front at the hearing with Karl Eikenberry, the US ambassador to Afghanistan, despite a public disagreement between the two over the US war strategy.

'Unequivocal' support

Eikenberry, a retired general and former commander in Afghanistan, acknowledged that he had questioned the size of a planned troop increase but said he "unequivocally" supported the final decision.

Barack Obama, the US president, last week announced an increase of 30,000 troops to Afghanistan, as well as a target date of July 2011 to begin the gradual withdrawal of troops.

The decision fuelled concerns in Afghanistan and in neighbouring Pakistan, as well as in the US, that the Taliban could attack a reduced force in 18 months time.

McChrystal acknowledged that while the timeline for a handover to Afghans posed no military problem, the Taliban could try to misrepresent the date for propaganda purposes.

He said that coalition forces faced "a complex and resilient insurgency" and that the most difficult task would be improving the credibility of local and national government.

But he said he did not expect to ask for more forces within a year.

The US troop buildup begins next week when 1,500 Marines are due to arrive in the southern Helmand province.