Pakistan has told the US it risks losing an ally if it continues to accuse the country of playing a double game in the war against al-Qaeda, escalating the crisis in relations between the two countries.
Hina Rabbani Khar, the Pakistani foreign minister, was responding to comments by Admiral Mike Mullen, the US joint chiefs of staff, who said Pakistan military's spy agency, ISI, was closely tied to the Haqqani network, the most violent and effective faction allied with the Taliban in Afghanistan.
It is the most serious allegation levelled by the US against Pakistan since they began an alliance in the "war on terror" a decade ago.
"You will lose an ally," Khar told Geo TV in New York in remarks broadcast on Friday.
"You cannot afford to alienate Pakistan, you cannot afford to alienate the Pakistani people. If you are choosing to do so and if they are choosing to do so it will be at their [the United States'] own cost."
The Haqqani network is a "veritable arm" of Pakistan's military intelligence agency, Mullen said on Thursday in prepared remarks to the US Senate Armed Services Committee.
He said the agency supported the group in its attack last week on the US embassy and NATO headquarters in Kabul. "The Haqqani network ... acts as a veritable arm of Pakistan's Inter-Services Intelligence agency," Mullen said.
General Ashfaq Parvez Kayani, Pakistan's Chief of Army Staff, also termed Mullen's comments as "very unfortunate and not based on facts".
"This is especially disturbing in view of a rather constructive meeting with Admiral Mullen in Spain," Kayani said in a statement.
"Categorically denying the accusations of proxy war and ISI support to Haqqanis," Kayani wished that "the blame game in public statements should give way to a constructive and meaningful engagement for a stable and peaceful."
The Haqqani network, which is distinct from the Afghan Taliban, is a member of the allied factions that are fighting the US and NATO troops in Afghanistan.
"With ISI support, Haqqani operatives planned and conducted [a September 11] truck bomb attack, as well as the assault on our embassy," Mullen said.
"We also have credible intelligence that they were behind the June 28 attack against the Inter-Continental Hotel in Kabul and a host of other smaller but effective operations."
Pakistan has denied the allegations.
Such attacks have been a blow to the US plan to withdraw its troops from Afghanistan, which is due to begin this year and end in 2014.
Also speaking at the hearing, Leon Panetta, the US defence secretary, said that these sorts of attacks were a sign of "weakness" on the part of fighters in Afghanistan.
"Overall, we judge this change in tactics to be a result of a shift in momentum in our favour and a sign of weakness in the insurgency," Panetta told the committee.
Just two days after the assassination of Burhanuddin Rabbani, the former Afghan president who was playing a role in mediating with the Taliban, Panetta said that as "the Taliban lost control of territory last year they shifted away from large attacks on our forces to greater reliance on headline-grabbing attacks".
Panetta said that the best signal that the US could send was that they would continue to fight the Taliban, and that this would convince them to negotiate a peaceful settlement.
Mullen, who is to step down this month from his post, agreed with Panetta's assessment. He said that while headline-grabbing attacks were "serious and significant in shaping perceptions ... they do not represent a sea change in the odds of military success".
Panetta claimed that security was improving across Afghanistan, particularly in areas where the US has increased troop numbers, though he did not go into specifics.
"While overall violence in Afghanistan is trending down, and down substantially in areas where we concentrated our surge, we must be more effective in stopping these [high-profile] attacks and limiting the ability of insurgents to create perceptions of decreasing security," he said.
He said the US military is "working with our Afghan counterparts to discuss with them how we can provide better protection against these attacks. But the bottom line is that we can't let these sporadic events deter us from the progress that we've made".
The comments on Pakistan's alleged links to the Haqqani group come after a series of tough statements from US officials on the subject, but were the toughest yet from Mullen.
"In choosing to use violent extremism as an instrument of policy, the government of Pakistan - and most especially the Pakistani Army and ISI [intelligence service], jeopardises not only the prospect of our strategic
partnership, but also Pakistan's opportunity to be a respected nation with legitimate regional influence," Mullen said.
"By exporting violence, they have eroded their internal security and their position in the region. They have undermined their international credibility and threatened their economic well-being."
Panetta echoed that sentiment, and when questioned by Carl Levin, the chairman of the committee, on what the US could do in addition to applying diplomatic pressure, he declined to share "operational details" on what could be done.
"I don't think [Pakistani leaders] would be surprised by the actions we might or might not take," he said.
Yet Mullen stressed that Pakistan had a crucial role to play in the achievement of US objectives in the region.
"A flawed and difficult relationship [with Pakistan] is better than no relationship at all," he said, adding praise for Pakistani authorities for having targeted al-Qaeda leaders in the country.
Senator John McCain, the ranking member of the committee, said that while Panetta and Mullen may be continuing efforts to achieve US objectives with regards to Pakistan through negotiations, it would be an "uphill battle" to convince the US Congress to continue its current levels of aid and funding to the country.