White House Chief of Staff William Daley on Sunday said the United States will withhold $800 million in military aid to Pakistan, further evidence of the strained ties between the two countries.
Mr. Daley, speaking on ABC's "This Week," said the two countries have a "complicated" relationship that the United States is committed to improving.
"They have taken some steps that have given us reason to pause on some of the aid which we were giving to their military, and we're trying to work through that," Mr. Daley said.
"The Pakistani relationship is difficult, but it must be made to work over time. But until we get through these difficulties, we will hold back some of the money the American taxpayers have committed to giving to them," he said.
Mr. Daley lauded the Pakistanis' anti-terrorism efforts, but the ties between the two countries have frayed since U.S. forces killed Osama bin Laden in Pakistan in May.
The suspension of U.S. aid 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 Adm. Mullen's comments and the suspension of aid represent "the end of happy talk" by which the U.S. tries to paper over differences between the two nations.
Defense Secretary Leon E. 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 Qaeda'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," Mr. 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 to terrorists in 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 fatally shot two Pakistanis who he said were trying to rob him. Tensions spiked in May, when U.S. forces killed bin Laden during a covert raid on a home in Abbottabad, the site 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 Pakistani sovereignty.
The $800 million in suspended aid represents 40 percent of the $2 billion in U.S. military aid to Pakistan.
Pakistan army spokesman Maj. Gen. Athar Abbas declined comment on the withholding. He pointed to comments by army chief Ashfaq Parvez Kayani, who last month said U.S. military aid should be diverted to civilian projects.
Hasan-Askari Rizvi, a Pakistani political and defense analyst, said the U.S. decision to suspend aid is an attempt to increase pressure on Pakistan, but he believes it could hurt both sides.
"The Pakistani military has been the major supporter of the U.S. in the region because it needed weapons and money," said Mr. Rizvi. "Now, when the U.S. builds pressure on the military, it will lose that support."
He said the move could make it harder for the U.S. to push the Taliban into peace talks as a preparation for its withdrawal from Afghanistan. At the same time, he said, the Pakistani military relies on U.S. aid in its fight against militant groups.