The United States says Al Qaeda's interest in obtaining nuclear weapons is still strong, and the risk of nuclear terrorism is serious and growing.

Ahead of a 47-nation nuclear security summit in Washington, American officials say they believe they have so far thwarted Osama bin Laden's group's efforts to build an improvised nuclear explosive device.

But they will not disclose whether new intelligence existed on Al Qaeda's most recent efforts to build an atomic bomb or whether it had made progress in exploiting unsecured nuclear material.

"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, US president Barack Obama's top anti-terrorism and homeland security adviser.

Mr Brennan says he has "no indication that Al Qaeda has a nuclear weapons capability," and adds that he is "determined" that the group never get such a capability.

"The threat of nuclear terrorism is real; it is serious; it is growing," he said.

Representatives from 47 countries are attending the two-day summit on nuclear security, which will try to come up with an agreement on how to keep nuclear material out of the hands of terrorists.

Speaking at the start of the summit, United Nations secretary-general Ban Ki-moon expressed his support for the meeting's aims.

"Nuclear terrorism is one of the greatest threats we face today," he said.

"That is why I repeatedly urged the conference on disarmament to immediately start negotiations on a treaty banning the production of fissile materials for nuclear weapons and other explosive devices."

- AFP/BBC