The ISI specifically directed the Haqqani network to carry out an attack last week on the US embassy and a NATO headquarters in Kabul, according to the US.
US officials said there was mounting evidence that Pakistan's powerful intelligence agency had encouraged a guerrilla network to attack US targets, while a Senate committee voted to make aid to Islamabad conditional on fighting the militants.
The decision by the Senate Appropriations Committee, which did not specify any amount of aid for Pakistan in fiscal 2012, reflects growing anger in Washington over militants operating out of Pakistan and battling US troops in Afghanistan.
Some US intelligence reporting alleges that Pakistan's Inter Services Intelligence directorate (ISI) specifically directed, or urged, the Haqqani network to carry out an attack last week on the US embassy and a NATO headquarters in Kabul, according to two US officials and a source familiar with recent US-Pakistan official contacts.
The Haqqani network is one of three, and perhaps the most feared, allied insurgent factions fighting US-led NATO and Afghan troops under the Taliban banner in Afghanistan.
However, US officials cautioned that the information that Pakistan's spy agency was encouraging the militants was uncorroborated.
Admiral Mike Mullen, chairman of the Joint Chiefs of Staff, said he had pressed Pakistan's army chief for Islamabad 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."
A Pakistani official sought to play down the differences.
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