WASHINGTON, Nov 28: The White House said on Monday that it understood why Prime Minister Yousuf Raza Gilani said that there would be “no more business as usual” with Washington but it still wanted to carry forward this important relationship.

And the Pentagon said the US military would hold a separate investigation to determine what caused US and Nato aircraft to attack Pakistani positions. Nato is already holding an inquiry.

Prime Minister Gilani's remarks that Pakistan was re-evaluating its relationship with the United States after Saturday's Nato air strikes echoed at a White House briefing where a reporter asked if the United States saw this statement as a threat.

“Well, we take the matter very seriously. And we understand concern that members of the Pakistani government, as well as the Pakistani people, have about this incident. And that's why we're very interested in having it investigated and finding out exactly what happened,” said White House Press Secretary Jay Carney.

Mr Carney noted that earlier in the day, the Pentagon asked the US Central Command to hold a separate inquiry because “we're obviously very keen on finding out exactly what happened”.

When a reporter asked how seriously was the White House taking Prime Minister Gilani's remarks, Mr Carney noted that the US-Pakistan relationship has been complicated for a long time, but “it is also an important relationship and one that we need to work hard on because that cooperation is in the interest of the United States”.

Asked how President Barack Obama reacted upon learning of the Nato strikes, the White House spokesman said Mr Obama's reaction was the same as that of others in his administration that the event that took those lives was a tragedy. “The loss of Pakistani life was a tragedy. We mourn the brave Pakistani service members who lost their lives and our sympathies go out to their families and go out to Pakistan,” he added.

Mr Carney noted that senior US officials had stayed engaged with their counterparts in Pakistan, including Secretaries Loen Panetta and Hillary Clinton, and those contacts would continue.

“It is very much in America's national security interest to maintain a cooperative relationship with Pakistan because we have shared interests in the fight against terrorism, and so we will continue to work on that relationship,” he said.

Mr Carney, however, said that President Obama had not yet spoken to Pakistani leaders on this issue, although “some of the most senior members on his national security team have”.

Asked if the situation had reached a crisis level, the White House said that this was “obviously a significant issue that we take seriously, and that's why we, through Centcom, will be investigating to find out exactly what happened”.

The Obama administration, however, believed that it would be in not just the United States' interest but Pakistan's interest too “to work with us cooperatively on our shared goals”.

Pakistan and the Pakistani people have been primary victims of terrorism, the US was working with them in dealing with this problem and this cooperative relationship had borne fruits for both.

At the Pentagon, Press Secretary George Little told reporters that the US was also to participate in a joint investigation with Pakistan.

The Centcom probe, he added, would supplement the findings of an initial assessment team sent to the scene by Gen John Allen, the top US and Nato commander in Afghanistan. “The expectation is that Centcom will lead the investigation,” he said.

The Pentagon spokesman indicated that some details of a preliminary investigation in Afghanistan were likely to be shared by Tuesday.

He said he was not aware of any disciplinary action against anybody in this case yet.

The Pentagon, he said, was aware of Pakistan's demand to vacate Shamsi airbase, but the US had no military personnel there at this point.

So far Pakistan has not asked the US to reduce its military presence in Pakistan, he added. The US Defence Department also knew that Pakistan had blocked Nato supply routes and was working to develop a way ahead and move beyond this incident with Pakistan. “We are looking at other supply routes too, because war efforts in Afghanistan against the enemy continue.”

Mr Little said he could not say if US aircraft crossed into Pakistan.

Top US generals, he said, had spoken to the Pakistan military chief and assured him that the US remained committed to relationship with the Pakistani despite these complications.