Washington ? The Obama administration’s decision to suspend $800 million in aid to the Pakistan’s military signals a tougher U.S. line with a critical but sometimes unreliable partner in the fight against terrorism.

President Barack Obama’s chief of staff, William Daley, said in a broadcast interview Sunday that the estranged relationship between the United States and Pakistan must be made “to work over time,” but until it does, “we’ll hold back some of the money that the American taxpayers are committed to give” to the country’s powerful military forces.

The suspension of U.S. aid, first reported by The New York Times, followed a statement last week by Adm. Mike Mullen, chairman of the U.S. Joint Chiefs of Staff, that Pakistan’s security services may have sanctioned the killing of Pakistani journalist Saleem Shahzad, who wrote about infiltration of the military by extremists. His battered body was found in June.

The allegation was rejected by Pakistan’s powerful military establishment, including the Inter-Services Intelligence Agency, which has historic ties to the Taliban and other militant groups and which many Western analysts regard as a state-within-a-state.

George Perkovich, an expert on Pakistan with the Carnegie Endowment for International Peace in Washington, said Mullen’s comments and the suspension of aid represent “the end of happy talk,” where the U.S. tries to paper over differences between the two nations.

Daley, interviewed on ABC’s “This Week,” suggested the decision to suspend military aid resulted from the increasing estrangement between the U.S. and Pakistan. “Obviously there’s still a lot of pain that the political system in Pakistan is feeling by virtue of the raid that we did to get Osama bin Laden,” Daley said.

Defense Secretary Leon Panetta told reporters traveling with him to Afghanistan on Saturday that the U.S. would continue to press Pakistan in the fight against extremists, including al-Qaida’s new leader, Ayman al-Zawahri.

“We have to continue to emphasize with the Pakistanis that in the end it’s in their interest to be able to go after these targets as well,” Panetta said. “And in the discussions I’ve had with them, I have to say that, you know, they’re giving us cooperation in going after some of these targets. We’ve got to continue to push them to do that. That’s key.”

The U.S. has long been unhappy with Pakistan’s evident lack of enthusiasm for carrying the fight against terrorists to its tribal areas, as well as its covert support for the Taliban and anti-Indian extremist groups.

But tensions ratcheted up in January, when CIA security contractor Raymond Davis shot and killed two Pakistanis who he said were trying to rob him. They spiked in May, when U.S. forces killed bin Laden during a covert raid on a home in Abbottabad, the location of Pakistan’s military academy.

In the U.S., there was anger at the possibility that some Pakistan officials had harbored the terrorist leader. In Pakistan, there was outrage that the U.S. operation had violated its sovereignty.