"I think the Al Qaeda is looking for those vulnerabilities and facilities and stockpiles in different countries that would allow them to obtain the by-product of nuclear reactors and materials that they can use," John Brennan, the White House's counter-terrorism chief, told reporters Monday.
The terror group was also looking "to go after those individuals that might have access to the materials as well as individuals who have the expertise that they need to actually fabricate and improvise nuclear devices," he said in a briefing on the Nuclear Security Summit here.
"There is evidence of their attempts to do that. I would like to think that we have been able to thwart their success to date," he said when asked if there is any evidence that Al Qaeda has actually managed to do that, particularly in Pakistan.
Seeking to highlight the urgency of the threat posed by terrorists in pursuit of a nuclear bomb, Brennan said that Al Qaeda is actively in search of the key ingredients for a bomb and the expertise to assemble it.
"Terrorist groups and internal criminal organisations will look for the weakest link in a chain and that's why it's so critically important that all countries take their responsibility seriously," he said.
Brennan declined to go into details of Sunday's meeting between President Barack Obama and Pakistani Prime Minister Yusuf Raza Gilani on the issue.
But "It's part of a process that was started, at least in this administration 15 months ago, where we had regular ongoing conversations with a number of nations of the world, including Pakistan, addressing the goals and objectives that we know that the Al Qaeda is after and what type of threat they pose to our interests and to the interests of other countries," he said.
"So, our engagement with Pakistan, runs the full gamut as far as what the Al Qaeda is trying to do whether it be to kill innocents or to carry out other types of attacks and objectives that really threaten our national security and of Pakistani national security," he said.
Asked about the danger of Pakistan's nuclear assets falling into the hands of the Al Qaeda, Brennan said: "The ability of a terrorist group to find weapons or expertise is directly related to the vulnerability of these materials."
Brennan said, "Over the years, the Al Qaeda, including some Al Qaeda members, have claimed that they already have such nuclear capabilities or weapons. That's not proved, but also at the same time, it's difficult to disprove something like that."
"There is no indication that I have that the Al Qaeda has a nuclear weapons capability. But at the same time though, I am determined to ensure that they are not going to be able to obtain that type of capability."