The United States has accused Pakistan's intelligence agency of using the Haqqani Network to wage a "proxy war", hardening its criticism of Islamabad's ties with Taliban-allied factions fighting NATO and Afghan troops in Afghanistan.
Admiral Mike Mullen, chairman of the Joint Chiefs of Staff, said that in a discussion with Pakistan's army chief that lasted about four hours, he had pressed Pakistan to break its links with the militant group.
"We covered ... the need for the Haqqani Network to disengage, specifically the need for the ISI to disconnect from Haqqani and from this proxy war that they're fighting," he said in a speech to the Carnegie Endowment for International Peace on Tuesday.
"The ISI has been doing this - working for - supporting proxies for an extended period of time. It is a strategy in the country and I think that strategic approach has to shift in the future."
Washington blames the Haqqani Network, one of the most feared Taliban-linked groups fighting in Afghanistan, for last week's attack on the US embassy and other targets in Kabul.
It has in the past suggested that Pakistan's powerful Directorate of Inter-Services Intelligence (ISI) maintains ties to the network to
guarantee itself a stake in any political settlement in Afghanistan when American troops withdraw.
Accusing the ISI of using the Haqqanis to wage a "proxy war" goes further, and risks fuelling tension between Islamabad and Washington, which have been running high since al Qaeda leader Osama bin Laden was killed in a surprise US Navy SEALs raid in Pakistan in May.
"In the past, they have been saying that Pakistan is looking the other way with the Haqqanis, but this term - using them as proxies for Pakistani interests - that is something new," said Rahimullah Yusufzai, editor of the Peshawar edition of the News daily and an expert on Afghanistan.
The Haqqani network is perhaps the most divisive issue between Pakistan and the United States. Washington has repeatedly pressed Pakistan to go after the network, which it believes enjoys sanctuaries in Pakistan's unruly ethnic Pashtun tribal region of North Waziristan on the Afghan border.
Pakistan denies accusations it has ties to the Haqqanis.
The Washington Post reported that US officials had delivered an ultimatum to Islamabad in recent days, warning that if it did not cut ties with the Haqqani Network and help eliminate its leaders then "the United States will act unilaterally".
"Look at the language, it's clear the Americans are very frustrated with the Pakistanis. I think they are preparing the ground for more action against the Haqqanis," Yusufzai said.
He said the United States could step up drone attacks from Afghanistan in North Waziristan or launch long-range attacks on Haqqani hideouts as they did in their helicopter raid on bin Laden in a town just two hours up the road from Islamabad.
Launching a larger military operation would be extremely difficult in the mountainous terrain of North Waziristan and would risk hardening anti-American sentiment in Pakistan.
There has been no public statement suggesting that the United States might itself mount a full-scale offensive against the Haqqanis in North Waziristan, and the official line in background briefings is only that all options are on the table.
A US Military Academy report in July said that the Haqqani Network was believed to be made up of several hundred core members who can draw on a pool of roughly 10,000 to 15,000 fighters.
The group's leader, Sirajuddin Haqqani, told Reuters last week that it no longer had sanctuaries in Pakistan, and instead felt secure inside eastern Afghanistan
While keeping the pressure on Pakistan over its links to insurgent groups, US officials are also trying to shore up relations with a nuclear-armed country it considers a strategic ally in the fight against Islamist militancy.
"What I believe is the relationship with Pakistan is critical," Mullen said. "We walked away from them in the past and ... I think that cut-off has a lot do with where we are."
A senior US official told Reuters on Tuesday that, despite the strains over last week's attack in Kabul, there had been incremental improvements in the relationship in recent weeks.
"I don't have a sense right now that its falling off the cliff again," he said.