The United States delivered a chilling warning Monday that Al-Qaeda's interest in nuclear weapons was still "strong" and said the risk of nuclear terrorism was "real," "serious" and "growing."

American officials said ahead of a 47-nation nuclear security summit here that there was intelligence that Osama bin Laden's group was continuing to pursue its "murderous agenda" by trying to obtain weapons of mass destruction.

But they did not characterize that intelligence, or disclose whether Al-Qaeda was making progress in stealing unsecured enriched uranium and plutonium stocks or was active on the nuclear black market.

"Al-Qaeda has been engaged in the effort to acquire a nuclear weapon for over 15 years, and its interest remains strong today," said John Brennan, President Barack Obama's top anti-terrorism and homeland security advisor.

Brennan said he had "no indication that Al-Qaeda has a nuclear weapons capability," and was "determined" that the group would never get one.

But he warned: "the threat of nuclear terrorism is real, it is serious, it is growing," adding that there was "indisputable" evidence that dozens of terrorist groups had sought weapons of mass destruction.

Brennan's comments reflected a subtext to the nuclear conference -- how realistic are dire warnings that terror groups could acquire the materials for a weapon, and then turn them into a deliverable atomic device.

In the same vein, Obama must use the meeting, which closes on Tuesday, to convince fellow world leaders, that their nations are just as at risk from the specter of nuclear terrorism as the United States.

Brennan stressed that the capacity of al-Qaeda to procure nuclear weapons was directly linked to the vulnerability of nuclear materials, so the best way to reduce the threat was to protect unsecured stockpiles.

His comments came as Obama prepared to welcome leaders and senior officials from 46 other nations to the nuclear security summit, and Ukraine announced it would dispose of all its highly enriched uranium by 2012.

Washington is trying to encourage other nations to take similar steps, and wants global powers to agree a set of concrete actions to safeguard nuclear material in weapons, nuclear reactors at at other sites.

Obama on Sunday conjured up the dire prospect of a nuclear attack on New York, London or Johannesburg, and warned that Al-Qaeda would have "no compunction" against using a device that could hundreds of thousands of people.

The thought of nuclear terrorism has worried US policymakers for years, but the threat took on horrific new dimensions following the September 11 attacks in 2001 -- the worst-ever strike on US soil.

Brennan would not say whether the threat posed by vulnerable nuclear stocks had got worse in recent years, but said the increasing prevalence of nuclear power generation could prove a headache in the coming years.

"As you have an expansion of nuclear programs, peaceful programs, there is going to be an increase in the nuclear by-products that come out of those facilities, as well as the expertise that is available to run them."

"But the availability of this material is going to be a factor of how well we're able to plug those gaps, plug those holes and address the vulnerabilities that are out there."

A new Harvard University report published on Monday said that the threat still loomed large from nuclear terrorism, despite some progress in securing vulnerable materials.

The Securing the Bomb 2010 report, found there had already been 18 documented cases of theft or loss of plutonium or highly enriched uranium.

It noted a 2010 break-in by unarmed peace activists at a Belgian base where US nuclear weapons were reportedly stored and a 2007 armed attack on a South African site housing hundreds of kilograms of highly-enriched uranium.

"Nuclear terrorism remains an urgent danger to world security, and securing nuclear weapons and materials within four years would dramatically reduce the risk," said Associate Professor Matthew Bunn of Harvard University's Project on Managing the Atom, who authored the report.