The most senior US military officer has accused Pakistan’s spy agency of supporting the Haqqani group in last week’s attack on the US Kabul embassy.
“The Haqqani network… acts as a veritable arm of Pakistan’s Inter-Services Intelligence Agency,” Adm Mike Mullen told a Senate panel.
Some 25 people died in last Tuesday’s 20-hour attack on Kabul’s US embassy and other official buildings.
Pakistan’s interior minister earlier denied links with the Haqqani group.
Rehman Malik told the BBC Pakistan was determined to fight all militants based on its border with Afghanistan.
Pakistani officials have consistently denied links with militant groups.
US-Pakistan ties deteriorated sharply after the killing of al-Qaeda chief Osama Bin Laden on Pakistani soil by US commandos in May.
The Kabul attack on 13 September left 11 civilians dead, as well as at least four police and 10 insurgents.
These comments are just the latest and most extreme in a series of statements that will be seen in Pakistan as incendiary. They will generate concern in government circles as well as among the wider public.
Pakistanis have long been worried that the Afghan war is coming to their side of the border.
The Haqqani network – and Pakistan’s alleged relationship with it – has been a source of frustration for the US. But only today Pakistan’s interior minister denied any links. Pakistan will also be keen to remind people that it too is in the grip of terror.
In the 1980s when militants were fighting the Soviets in Afghanistan, the head of the Haqqani network was nurtured by Pakistani intelligence – and indeed by the CIA.
Some analysts believe the links between the militants and Pakistan’s intelligence are still alive. But others say that Pakistan’s secret service no longer has control over the potent militant groups it helped created.
“With ISI support, Haqqani operatives planned and conducted a truck bomb attack [on 11 September], as well as the assault on our embassy,” said Adm Mullen.
“We also have credible intelligence that they were behind the 28 June attack against the Inter-Continental Hotel in Kabul and a host of other smaller but effective operations.”
In July Adm Mullen, who steps down this month as chairman of the US Joint Chiefs of Staff, accused Pakistan’s government of sanctioning the killing of investigative journalist Saleem Shahzad.
Pakistan called that statement “irresponsible”.
Correspondents say that during his tenure, Adm Mullen has been a forceful advocate for maintaining dialogue with Pakistan and with its military establishment.
He was said to be close to the Pakistani army’s chief of staff, Gen Ashfaq Kayani. Indeed, Adm Mullen is thought to have made more visits to Pakistan than any other senior US official or chief of staff in recent times.
But, correspondents say, the latest comments are yet more evidence of his patience wearing thin, and suggest he is prepared to be more outspoken as his term in office draws to a close.
The Haqqani network, which is closely allied to the Taliban and reportedly based in Pakistan, has been blamed for several high-profile attacks against Western, Indian and government targets in Afghanistan.
It is often described by Pakistani officials as a predominantly Afghan group, but correspondents say its roots reach deep inside Pakistani territory, and speculation over its links to Pakistan’s security establishment refuses to die down.
US officials have long been frustrated at what they perceive to be Pakistani inaction against the Haqqani network, and analysts say US concern about the group’s capabilities is particularly acute as Nato begins withdrawing troops from Afghanistan.
Earlier this month, Washington said it could target the Haqqani network on Pakistani soil if the authorities there failed to take action against the militants.
But on Thursday, Mr Malik told the BBC that Pakistan’s government had taken “very, very strict actions” whenever it had received information about militant groups.
“We will not allow any terrorist to operate from our area, from our side, irrespective of any country, including Afghanistan,” he said. “I assure you that, if their presence is there and which is detrimental, action is going to be taken.”
Mr Malik said his government’s efforts were hindered by the fact that neither Pakistan nor Afghanistan had control over some parts of the border area between them.
“There is no biometric system on the border. Forty thousand to 50,000 people cross this border every day. It is very difficult to keep an eye on everyone.
“[The US Senate’s linking of $1bn US assistance to Pakistan to its action again Haqqani and others] will not make Pakistanis happy,” he added. “We have sacrificed 35,000 lives [in fighting terrorism] and have suffered economic losses.”
Ties between the US and Pakistan had already been strained by continuing US drone strikes targeting militants in the tribal areas and the controversy over the release of Raymond Davis, the CIA contractor who killed two Pakistani men in Lahore.