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 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.

Afghan officials say mobile phones found on the slain attackers in this week's commando-style raid in Kabul indicate they were in contact with people from 'outside Afghanistan' -- a typical Afghan way of indirectly pointing to Pakistan.

Even so, US and Afghan officials have stopped short of publicly linking the attack to the ISI, as they did after past attacks in Kabul, such as the 2008 and 2009 bombings of the Indian Embassy in Kabul.

In those and other cases, US officials said that communications intercepts and other intelligence directly linked the ISI to the attacks.

Yet it took months to reach that conclusion and publicise it. What is different this time is the speed with which some US officials publicly said they were exploring ISI links, a sign of the growing frustration of US officials who in recent months have become more public in their finger-pointing at Pakistan for its coordination with Islamist militant groups.

According to the daily, a senior US defence official said there is currently no "actionable intelligence" linking Pakistan's spy service to this week's attack. "But we're looking for it closely," the defence official said shortly after the violence ended," the official was quoted as saying.

"That illustrates the deep vein of mistrust now running through the relationship between Washington and Islamabad," the officials said.

"The level of patience has just gone out the window," Seth Jones, a political scientist at the Rand Corp, who has spent much of the past two years working with the US military in Afghanistan, was quoted as saying.

"People aren't keeping it inside anymore and containing it in a circle that, for a while, was just private," he said.

"US officials say they are looking for evidence that directly links elements of Pakistan's powerful spy agency to this week's assault on the US Embassy and coalition headquarters in Kabul, a sign of just how rancorous relations have become between the two allies in the fight against al Qaeda and the Taliban," the daily said.

A direct attack on an American embassy "isn't something we can treat as business-as-usual," said the US defense official.

Even if no ISI link is found, the Pakistani relationship with the Haqqanis is 'long past unacceptable,' the official said.

"Missteps by both the US and Pakistan this year have led to a sharp deterioration in relations, which may also be prompting Islamabad to more closely embrace militant groups from which it has sought to publicly distance itself in recent years, according to US officials and Mr Jones of Rand," the daily said.