Accusing Pakistan's military-run ISI of using the Haqqani network to carry out a ‘proxy war’, US has warned Islamabad to cut ties with the terror group and help eliminate its leaders or it will act unilaterally.

In what amounts to an ultimatum, the US administration have indicated that the US will act unilaterally if Pakistan does not comply, 'The Washington Post' daily reported.

Quoting officials, the paper said the message was delivered in high-level meetings and public statements over the past several days reflecting Washington's view that the year-long strategy of using persuasion and military assistance to influence Pakistan's behaviour has been ineffective.

According to the ‘Post’, White House officials and Defence Secretary Leon Panetta are said to be adamant in their determination to change the approach. The report comes as Chairman of the Joint Chief of Staff Admiral Mike Mullen told a meeting here last night that in his discussions with Pakistan army chief Gen Ashfaq Parvez Kayani, he had pressed Islamabad to end its links with the terror group.

"We covered a full range of issues focusing on the danger of the Haqqani network, the need for the Haqqani Network to disengage ... 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 during his address at Carnegie Endowment for International Peace.

"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," He added.

The meeting with Kayani that took place in Spain lasted for over four hours. The US blamed the Haqqani network, one of the most feared Taliban and al-Qaeda linked groups fighting in Afghanistan for last week's attacks on the US embassy and NATO headquarters in Afghan capital Kabul.

American officials have suggested in the past that ISI has maintained links with Haqqanis to guarantee that Islamabad has a stake in any future political settlement in Afghanistan once US troops withdraw.

Washington has also vowed to put "as much pressure as possible" on Islamabad to control these insurgent groups on their side of the border.

Meanwhile, while world leaders, jarred by the brazen assassination of top Afghan leader Burhanuddin Rabbani yesterday in Kabul, said his death would not derail Afghan peace process, the US military said the attack signalled that the Taliban was changing tactics by moving off the battlefield to go after high-profile targets elsewhere.

"Strategically, they're significant," said Mullen, referring to a series of Taliban suicide bombings over the summer that killed, among others, President Karzai's half brother and recent bold attacks near the US embassy and NATO headquarters in the Afghan capital.

Mullen and Defense Secretary Leon Panetta, who were speaking at a Pentagon press briefing here, said the assassination tactic was a "concern" and US commanders were working with Afghan forces to try to thwart the Taliban.

"The bottom line still remains that we are moving in the right direction," Panetta added.