The top United Nations expert on disarmament issues says terrorists could possibly obtain a miniature nuclear weapon - but that it would not be easy.

U.N. Undersecretary-General for Disarmament Jayantha Dhanapala told reporters Thursday it is true that nuclear weapons can be miniaturized, with the smallest atomic bomb that he is aware of weighing just 23 kilograms. However, Mr. Dhanapala said that even if a terrorist group could obtain the material for such a weapon, it would be difficult to assemble. "It is in fact possible to miniaturize nuclear weapons," he said. "But the extent to which they are available to terrorist groups is still an open question. There are a number of aspects that are necessary, not just the material but even the way in which it is assembled and of course the capacity to actually deliver such a 'suitcase' bomb."

Mr. Dhanapala said strengthening the existing international treaties on weapons proliferation is key to keeping weapons of mass destruction out of the hands of terrorists. For example, the U.N. official said, there is a loophole in an international agreement on the control of biological weapons. "The biological weapons convention for example, does not have any mechanism for verification," he said. "An ad hoc group that was working over a seven year period to try to develop such a protocol was not successful."

Mr. Dhanapala said an upcoming biological weapons review conference in Geneva next month may provide another opportunity to discuss a verification process.

As frightening as the idea of weapons of mass destruction may be, Mr. Dhanapala said terrorist access to conventional small arms and light weapons remains an ongoing problem.