WASHINGTON -- US President Barack Obama defended his Afghanistan policy on Monday, saying he continues to have "faith" in the US mission in the country.

"What's important is we got the policy right, but this is hard and it always has been," Obama told reporters at the White House, rejecting criticism from former Pentagon chief Robert Gates.

Gates, who was defense secretary from 2006 to 2011, questioned in a new memoir whether Obama supported his own policy in Afghanistan.

"Just as I have continued to have faith in our mission, more importantly I have had unwavering confidence in our troops," Obama said.

The failure of the Afghan government to sign a bilateral security agreement with the United States has strained ties between the two countries.

The White House has urged Afghan President Hamid Karzai to sign the agreement by the end of 2013 to give enough time for planning and deciding the number of troops to remain in Afghanistan after 2014.

However, Karzai refused to sign the accord until after a new president is elected on April 5, 2014, or unless Washington meets preconditions including immediately halting US raids on Afghan residences, ensuring viable security and supporting meaningful peace talks with the Taliban.

In another row, the United States on Thursday voiced opposition to Afghanistan's latest decision to release 72 of 88 suspected Taliban inmates, calling them "dangerous criminals."

Washington said the 88 inmates are suspected to be involved in the deaths of 117 members of the Afghan or international security forces.