US suspects that Pakistan's powerful spy agency ISI could be behind the attack on its embassy in Kabul this week and is looking for direct evidence in this regard.
A senior US defence official said that given the ISI's history of supporting and sheltering the Haqqanis, it was "almost reflexive" to see if the spy agency had any role in the latest Kabul violence that left 27 people dead.
"The possibility of ISI involvement was already being considered within hours of the attack's conclusion when President Barack Obama's National Security Council met on Wednesday, said a US official," The Wall Street Journal reported on Saturday.
The American suspicions are being partly fuelled by growing concerns that deteriorating bilateral relations, and the withdrawal of troops from Afghanistan, may be pushing elements of Pakistan's Inter-Services Intelligence spy agency to more closely embrace the Haqqani network, the Taliban faction blamed for this week's violence and a spate of attacks in and around Kabul.
Neither the ISI nor the Pakistani military, of which the spy agency is part, immediately responded to the US suspicions, the WSJ said, adding that Pakistani government officials dismissed the suspicions as insulting and unfair.
Top US officials, including Defence Secretary Leon Panetta, have already blamed the violence in Kabul on the Haqqani network, an Afghan insurgent faction whose history is intertwined with the ISI.
The Pakistani spy agency has aided Haqqani network attacks in Kabul in past years, officials say.
The US has warned the Pakistanis of stronger action if the group wasn't reined in.