Sending a clear message to Pakistan, the United States on Friday said terror groups in that country has to be 'shut down' and assured India of help in getting terrorists behind the Mumbai attacks.

Coverage: Mumbai terror attacks

"We want to make sure that the groups which originated these attacks from Pakistan are shut down and put out of business and brought to justice," US Assistant Secretary of State Richard Boucher told media persons in Mumbai.

"It is a long-term thing that needs to be done thoroughly and consistently. That is what we are doing," he said.

On a brief visit, Boucher promised US help on being asked if it would help get terrorists wanted by India from Pakistan. "You can rely on the United States," he responded.

Boucher said Pakistan 'needs to get rid of all terrorist groups'.

"You cannot have one group operating in Kashmir and another in Mumbai. It is legitimate to close down these places and that is what we want to ensure," Boucher said, when asked if India were to carry out surgical strikes against terror groups across the border.

"None of us, Including Pakistan, India and US, is safe as long as terror groups are tolerated," he said.

To a question if Pakistan's nuclear capability was a factor to be considered for India to formulate its response to the 26/11 strikes, Boucher said: "I don't think that Pakistan having nuclear weapons is the most important factor."

Stating that the probe into Mumbai attack was an Indian investigation and that the US personnel are in India primarily in a supportive role, Boucher said "it is a big task".

"We really need to get to the bottom of this. We want to know who did it, how they did it. Who else might be still out there trying to do a similar thing," he stressed.

Asked if the 26/11 attack could be called an act of war by Pakistan, Boucher said, "We know that history of terrorism in this region and we also know how some of these terror groups have been supported by state powers."

"We also know that we are in a time now when it should not be tolerated. None of us, including Pakistan, are safe as long as such groups are tolerated."

"It is too early to talk about investigative conclusions. It is premature to express any conclusions," Boucher said when asked if the terror probe had yielded information about who was behind them.

The American official said Pakistan had made a 'promising start' in the fight against terror but there is a lot that still needs to be done.

"We need to ensure that attacks like 26/11 don't recur," he said.

On Kasab's [Images] nationality, Boucher said: "I don't know his nationality myself. I am sure the investigators do. I have talked to FBI guys. We all know that this attack had originated from Pakistani soil. We have to follow the traces of how this was done, trace it back to groups in Pakistan.

"That is how the investigation needs to proceed. We need to know who these people were, how were they organised and eliminate these groups," he said.

Asked what would be the US role if it is proved that Kasab is a Pakistani national, Boucher said: "Our role is what it is now and what it is going to be. We expect all sides, including Pakistan, to investigate this thoroughly and punish those involved."

Asked if the US would put pressure on Pakistan to hand over underworld don Dawood Ibrahim [Images] to India, Boucher said. "He is a wanted man, but I think what we are dealing with right now is the Mumbai situation from these attacks. There are a lot of criminals we would like to be brought to justice."

Asked if it would be legitimate on part of Indian defence forces to attack 'terror factories' in Pakistan, Boucher said: "It is legitimate to close down these places and that is what we are trying to do. Let us understand that closing down these groups is something that Pakistanis have themselves said they would like to do and we should expect them to carry it out."        

On whether India's response to the 26/11 attacks is a 'sign of maturity or spinelessness', Boucher said: "I would say India's responses are responses we would cooperate with and support."