The US is boosting its troop numbers in a bid to counter growing Taliban violence across the country, following orders from Barack Obama, the US president, who has made success in Afghanistan a central tenet of his foreign policy.

At present there are 54,000 US troops in Afghanistan and that figure is expected to reach 68,000 later this year.

Other nations, mainly Nato members, have more than 30,000 troops in the country.

McChrystal has spent the past 10 months or so in a senior Pentagon desk job as director of joint staff, but ran the secretive Joint Special Operations Command from 2003 to 2008.

During this period, troops who worked for the command reportedly committed abuses including beating detainees with rifle butts, simulating drowning, and using them for target practice in paintball games, US media reports said.

The allegations led rights groups to express concern over McChrystal's nomination.

Speaking for the first time in public about the issue, McChrystal told senators that he did not condone the mistreatment of detainees and that those involved in the allegations were punished.

He said he believed that methods of interrogation had stayed within "established and authorized guidelines" set by Donald Rumsfeld, then secretary of defense.

He said he would "strictly enforce" the highest standards with regard to the treatments of detainees in a manner "consistent with international and US law".

McChrystal is widely expected to be confirmed when his nomination is put to a later vote in the house and senate.