WASHINGTON - US President Barack Obama defended his Afghanistan policy, saying he continues to have "faith" in the U.S. mission in the country, after his former Pentagon chief said the president had changed course on his own decisions.

"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.

Obama insisted he had faith in the Afghan war mission after Gates charged the president lacked passion for military action and soured on his own troop surge.

In his first public comments on criticisms of his role as commander-in-chief in a new memoir by Gates, Obama said he had a duty to constantly question US military tactics and "sweat the details" when sending young men and women to war.

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

But the president declined to say whether he had been irked by the publication of the book while he was still in office and American forces remained on the battlefield.

Gates sparked a Washington firestorm when he suggested in "Duty," to be published Tuesday, that Obama became disillusioned with the troop surge strategy launched in 2009 by early 2011, and lost confidence in his troop commanders and Afghan President Hamid Karzai.

The failure of the Afghan government to sign a Bilateral Security Agreement (BSA) with the United States has strained ties between the two countries.

The White House has urged 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 BSA until after a new president is elected on April 5, or unless Washington meets preconditions including immediately halting US raids on Afghan residences, ensuring viable security and supporting meaningful peace talks with the Taliban.

NATO forces are withdrawing from Afghanistan by the end of 2014, after which time the Afghan forces will take over full security responsibility. The alliance currently has around 80,000 troops in Afghanistan, the majority being American.

NATO plans to leave a training mission, expected to number 8,000 to 12,000 soldiers, in Afghanistan if the BSA is signed. (Tolonews)