The US military's top officer, Adm Mike Mullen, has accused Pakistan's spy agency of having links with militants targeting troops in Afghanistan.
He said Pakistan's Inter-Services Intelligence (ISI) had a "long-standing relationship" with a militant group run by Afghan insurgent Jalaluddin Haqqani.
The comments came as he held talks in Islamabad on Wednesday. Pakistani officials are also in the US for talks.
Pakistan routinely rejects charges of collusion with militants.
The BBC's M Ilyas Khan in Islamabad says that US officials have in the past spoken anonymously or in circumspect terms about associations between the Pakistani establishment and insurgents.
But that with this blunt statement Adm Mullen has for the first time claimed a clear link between the two, our correspondent says.
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