The Obama administration for the first time Thursday openly asserted that Pakistan was indirectly responsible for specific attacks against U.S. troops and installations in Afghanistan, calling a leading Afghan insurgent group “a veritable arm” of the Pakistani intelligence service.

Last week’s attack on the U.S. Embassy in Kabul and a Sept. 10 truck bombing that killed five Afghans and wounded 77 NATO troops were “planned and conducted” by the Pakistan-based Haqqani network “with ISI support,” said Adm. Mike Mullen, chairman of the Joint Chiefs of Staff. The ISI is the Pakistani military’s Inter-Services Intelligence agency.

“The government of Pakistan and most especially the Pakistani army and ISI” have chosen “to use violent extremism as an instrument of policy” to maintain leverage over Afghanistan’s future, Mullen testified during a hearing of the Senate Armed Services Committee. Defense Secretary Leon E. Panetta also testified.

Mullen’s statement represented a sharp break with a long-standing administration policy of publicly playing down Pakistan’s official support for Taliban insurgents who operate from havens within its borders. U.S. officials have typically described Pakistan as a troublesome but valuable partner in the struggle against terrorism.

The testimony capped a week of increasingly critical administration statements in the wake of the recent attacks and reflected a rising conviction that a new strategy is needed.

Pakistani Foreign Minister Hina Rabbani Khar condemned Mullen’s allegations and issued what sounded like a veiled counter-threat, warning that the United States could ill afford to risk its relationship with Pakistan.

“If they are choosing to do so, it will be at their own cost,” Khar told the Pakistani television network Geo on Thursday from New York City, where she is attending a U.N. General Assembly meeting. “Anything which is said about an ally, about a partner publicly to recriminate it, to humiliate it is not acceptable,” she said.

Even as they denounced Pakistan, Mullen and Panetta insisted that the recent attacks, among the most brazen of the 10-year-old war, were an indication of increasing Taliban desperation as U.S. military pressure has diminished the insurgents’ ability to conduct all-out offensives. On Tuesday, a suicide bomber with explosives concealed in his turban killed former Afghan president and leading peace negotiator Burhanuddin Rabbani in Kabul.

Rabbani’s killing is under investigation, said a U.S. official with access to intelligence from Afghanistan. He said he is “not aware” of any information linking the Haqqani group to that attack.

Mullen and Panetta deflected lawmakers’ questions about what actions the administration is prepared to take to stop Pakistan’s support for insurgents.

“We’ve made clear that we are going to do everything we have to do to defend our forces,” Panetta said. “I don’t think it would be helpful to describe what those options would look like and what operational steps we may or may not take.”

“I think the first order of business right now is to, frankly, put as much pressure on Pakistan as we can to deal with this issue from their side,” Panetta added.

The administration has said that “credible intelligence” shows that the Sept. 13 embassy attack, the truck bombing in nearby Wardak province and a June 28 attack on the Intercontinental Hotel in Kabul were conducted by the group led by Sirajuddin Haqqani, based in the Pakistani tribal region of North Waziristan. U.S. military officials have said that the group, part of a number of Taliban affiliates with havens in Pakistan, poses the greatest threat to American troops in Afghanistan.

In meetings over the past week with Pakistan’s military and intelligence chiefs, and with the country’s foreign minister, President Obama’s top national security officials have warned that U.S. tolerance has reached the breaking point.

The statement Thursday was especially significant because it came from Mullen, who has been the administration’s point man for building relations with the Pakistani military and has met dozens of times in recent years with the Pakistani army’s chief, Gen. Ashfaq Kayani.

“I’ve done this because I believe that a flawed and difficult relationship is better than no relationship at all,” Mullen said. “Some might say I’ve wasted my time, that Pakistan is no closer to us than before and may now have drifted even further away.”

Mullen said he disagreed with such critics, and he noted that “with Pakistan’s help, we have disrupted al-Qaeda and its senior leadership in the border regions.”

But Pakistan’s use of insurgent groups as “proxies” for leveraging influence in Afghanistan, he said, has “eroded their internal security and their position in the region.”

Pakistan’s public reaction to the warnings has been relatively subdued. Interior Minister Rehman Malik categorically denied ISI involvement in the embassy attack. “We have no such policy to attack or aid attacks through Pakistani forces or through any Pakistani assistance,” Malik told the Reuters news agency.

A Pakistani intelligence officer said in an interview Thursday in Islamabad, Pakistan’s capital, before Mullen’s testimony that the recent allegations had “dealt a severe blow” to U.S.-Pakistan relations, which had only recently begun to thaw after the unilateral U.S. military raid in May that killed Osama bin Laden in the Pakistani city of Abbottabad. The officer, who was not authorized to publicly present the official government position, said that “so far, U.S. authorities have not provided us with any tangible or specific evidence proving links between the ISI and the Haqqani network.”

“This is very frustrating for Pakistani security circles, and it is creating an impression that a case is being prepared against Islamabad in Washington,” the officer said.

U.S. officials — and some Pakistani officials — countered that the administration has repeatedly provided Pakistan with evidence of the whereabouts of the Haqqani leadership in Miranshah, North Waziristan’s largest city, including surveillance photographs of their headquarters in a former school and evidence of meetings with ISI officials.

CIA Director David H. Petraeus provided the ISI chief, Lt. Gen. Ahmed Shuja Pasha, with specific evidence of Haqqani involvement in the embassy attack when they met at agency headquarters Tuesday, according to a second official who was not authorized to discuss intelligence matters. No U.S. citizens were harmed in the attack, in which seven Afghans were killed.

The preferred administration option for dealing with Haqqani is for the Pakistanis to capture or kill the network’s top leadership or mount a joint operation with U.S. assistance, administration officials said. Second would be to provide intelligence to assist a U.S. ground operation or act to draw Haqqani leaders into unpopulated areas where drone strikes could target them.

Pakistan remains unsure whether the administration “has decided to take its words to their logical conclusion” and strike on its own, either with drones or ground forces, one senior Pakistani official said. Some civilians inside Pakistan’s government have pushed for a reevaluation of policy toward the Afghan insurgents, the official said, but the power structure remains divided and “there are people arguing for business as usual.”

Similar divides remain within the Obama administration, where stability in Pakistan is widely viewed as a key component of regional peace, although decreasing numbers appear to be advocating more patience.

Although some lawmakers noted the sacrifices Pakistan has made in battling domestic insurgents and in capturing and assisting with drone targeting of al-Qaeda leaders, others warned of cuts in and conditions on U.S. military and economic assistance.

“As far as I’m concerned,” Panetta said of conditional aid, “anything that makes clear to them that we cannot tolerate their providing this kind of safe haven to the Haqqanis and that they have to take action — any signal we can send to them, I think — would be important to do.”

Correspondent Karin Brulliard in Islamabad contributed to this report.