WASHINGTON—A new White House assessment steps up criticism of Pakistan's campaign against militants, stating bluntly that its government and military have been unwilling to take action against Al Qaeda and like-minded terrorists.

The aggressive language of the report—which also criticizes the leadership of President Asif Ali Zardari—could further strain difficult relations with a key ally and undercut support in Congress for providing billions of dollars in aid to Islamabad.

The report, viewed by The Wall Street Journal, also raises questions about the U.S.-led coalition's progress battling the Taliban and improving governance in Afghanistan two months before the White House will review its war strategy.

The administration and Pentagon have until now tried to keep their harshest criticisms of Pakistan private to avoid a public rift, but the report shows growing U.S. frustration, officials said. "The report reflects that there are real challenges we have with Pakistan," said an Obama administration official. Officials at all levels are in talks with Pakistan to address these issues, the official added.

President Barack Obama, in a letter to Congress accompanying the report, said he doesn't see the need for any adjustments in Afghanistan-Pakistan strategy "at this time."

While administration officials have publicly played down the need for adjustments in strategy, they have made some changes, including a recently stepped-up campaign of strikes in Pakistan by Central Intelligence Agency drones against militants whom the U.S. sees Islamabad as unable or unwilling to attack.

Pakistani officials have said they don't lack the will and that they have generally stepped up their efforts in response to U.S. requests, getting too little credit for it. But they say their army is already stretched thin—a problem exacerbated when soldiers were diverted to respond this summer to the worst flooding in the country's history.

"The Pakistan military continued to avoid military engagements that would put it in direct conflict with Afghan Taliban or Al Qaeda forces in North Waziristan," the White House concludes, referring to the Pakistani tribal region that U.S. officials say is being used as a staging ground for attacks on troops in Afghanistan, as well as to plot attacks on targets in Europe.

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