Washington, Apr 10 (IANS): Pakistan has been invited to the nuclear security summit despite being a "major source" of nuclear proliferation even as North Korea, Iran and Syria have been kept out as the US wants Islamabad to be part of the solution.

"We want to see Pakistan be part of the solution in the future. It has been part of the problem in the past," State Department spokesman P. J. Crowley told reporters Friday explaining the rationale of inviting Pakistan to the gathering of 47 world leaders.

"Pakistan has been a source of proliferation, and at various times in the past we have taken specific steps against Pakistan as a result," he acknowledged but suggested Pakistan had recently demonstrated its willingness to help the international community to shut down the A. Q. Khan network run by the notorious Pakistani scientist.

"You're quite right, Pakistan has been a source of concern in the past and we have had significant discussions with Pakistan on these issues," Crowley said when reminded that Pakistan is the country in which the greatest nuclear proliferation has ever taken place under A. Q. Khan.

"We still have questions about that and we still pursue those with Pakistan," he said. But "It has demonstrated that it can secure its own, nuclear weapons programme, and the US has confidence in the steps that Pakistan has taken."

"But if we're going to strengthen the non-proliferation regime going forward, we want to see, Pakistan invested in this process."

"To the extent that other countries demonstrate through their cooperation with the international community that they are willing to assume that same responsibility, then the door would be open for further cooperation," Crowley said.

On the other hand, Iran, North Korea and Syria were not invited to the Nuclear Security Summit "for their refusal to cooperate with the international community."

The governments that will be in attendance next week have shown a willingness to work cooperatively within the international community to strengthen, the security of nuclear weapons and nuclear know-how, he said.

"The three countries that you just cited - North Korea, Iran and Syria - have steadfastly either refused or failed to cooperate effectively with the International Atomic Energy Agency.

"So we are strengthening the non-proliferation regime expressly to be able to deal with, those countries that pose significant challenges, you know, to our long-term security," Crowley said.