There is evidence linking the Haqqani militant network to Pakistan's government, the US ambassador to Pakistan has said in a radio interview.
"This is something that must stop," Cameron Munter told Radio Pakistan, when discussing Tuesday's militant assault on the Afghan capital, Kabul.
At least 25 died during a 20-hour-long attack blamed on the Haqqani group, who are believed to be based in Pakistan.
Pakistani authorities have consistently denied links with militant groups.
The Haqqani network, which is closely allied to the Taliban, 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 refuse to die down.
"The attack that took place in Kabul a few days ago that was the work of the Haqqani network, and the fact that, as we have said in the past, that there are problems, there is evidence linking the Haqqani network to the Pakistan government, this is something that must stop," Mr Munter said in the interview.
Although US officials have long harboured suspicions about the alleged links, they rarely make such public and direct statements.
In July the top officer in the US military, Adm Mike Mullen, said the Pakistani government "sanctioned" the killing of investigative journalist Saleem Shahzad. The Pakistani government called that statement "irresponsible".
Analysts say that US officials have long been frustrated at what they perceive to be Pakistani inaction against the Haqqani network, thought to based in Pakistan's tribal areas.
Last week, Washington said it could target the Haqqani network on Pakistani soil if the authorities there failed to take action against the militants.
Ties between the uneasy allies deteriorated sharply after the killing of al-Qaeda chief Osama Bin Laden on Pakistani soil by US commandos in May.
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, had already strained ties.
Mr Munter acknowledged that relations had been "tough" and went on to say that it was time for the two countries to work together to defeat the militants.
"From our side we think that fighting them together is the best way to do it," he said.
But Sirajuddin Haqqani, the son of the leader of the network, has told the Reuters news agency that the group no longer has sanctuaries on Pakistani soil because it felt secure inside Afghanistan.
"Gone are the days when we were hiding in the mountains along the Pakistan-Afghanistan border. Now we consider ourselves more secure in Afghanistan besides the Afghan people," he said.
He also said that the group would take part in peace talks with Kabul and the US if the Taliban endorsed such talks as well.
Previously the group has rejected such overtures, the agency reported.
But he declined to comment on Tuesday's attack on Kabul saying that he had been instructed by senior leaders not to say anything if Western interests were attacked.
"For some reasons, I would not like to claim that fighters of our group had carried out the recent attack on US embassy and Nato headquarters," he said.
Reuters said they spoke to Sirajuddin Haqqani by telephone from an undisclosed location.
Analysts say US concern about the capabilities of the Haqqani network is particularly acute as Nato begins withdrawing troops from Afghanistan.