Barack Obama insisted Monday he had faith in the Afghan war mission after former Pentagon chief Robert Gates said 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 new memoirs by Gates, Obama said he had a duty to constantly question US military tactics and "sweat the details" when sending men and women to war.

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.

While praising Obama for his integrity and intellectual approach to matters of state, Gates is reported to have written that the president showed less zeal for military engagements than his predecessor George W Bush. "One quality I missed in Obama was passion, especially when it came to the two wars," Gates wrote, according to Fox News.

Obama, however, said that he "continued to have faith" in the US mission in Afghanistan.

"Most importantly, I've had unwavering confidence in our troops and their performance in some of the most difficult situations imaginable," the president said.

AFP