WASHINGTON The United States Friday defended its decision to invite Pakistan, a 'major source of nuclear proliferation in the past, to next weeks US-sponsored nuclear security summit, saying it wants Islamabad to be part of the solution. Questioned about the rationale for inviting 'proliferator Pakistan, the State Department told a news briefing, We want to see Pakistan be part of the solution in future. It has been part of the problem in the past. Pakistan has been a source of proliferation, and at various times in the past we have taken specific steps against Pakistan as a result. Assistant Secretary of State for Public Affairs P. J. Crowley reporters that Pakistan has now demonstrated its ability to protect its atomic programme and he drew attention to Islamabads cooperation in shutting down the A.Q. Khan network coupled with the safeguards put in place for its strategic assets. It (Pakistan) has demonstrated that it can secure its own nuclear weapons programme, and we have confidence in the steps that Pakistan has taken, he said. We still have questions about that and we still pursue those with Pakistan, Crowley said in response to a question. Youre quite right, Pakistan has been a source of concern in the past and we have had significant discussions with Pakistan on these issues. But if were 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. But in the case of the three countries that were noted earlier, they have noted right now for their refusal to cooperate with the international community. That was the reason why, Crowley said, three countries Iran, North Korea and Syria, were not invited to the Nuclear Security Summit. 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.