Mr ElBaradei said it was hard to stop the spread of nuclear technology

Mohamed ElBaradei, the head of the International Atomic Energy Agency (IAEA), said there were "temptations" for countries to develop nuclear bombs.

Last week, North Korea carried out its first successful detonation of a nuclear bomb.

It became the ninth country known to have a nuclear weapons capability.

Mr ElBaradei told a conference at the IAEA's headquarters in Vienna that 20 to 30 countries "have the capacity to develop nuclear weapons in a very short timespan".

"We are dealing with almost, as I call them, virtual nuclear weapons states," he said.

He said the lack of international security, and the failure of existing nuclear-armed states to dismantle their own arsenals, made it hard to persuade other countries not to work on nuclear programmes.

"Unfortunately, the political environment is not a very secure one... there are a lot of temptations (to develop nuclear weapons)," he said.

Expanding nuclear club

The conference will look at improved technology for detecting the signs of nuclear weapons development, including use of satellite imagery and sophisticated sampling.

Mr ElBaradei warned that so much information was now in the public domain that it was difficult to restrict the flow of nuclear technology.

Iran has been pursuing a uranium enrichment plan that diplomats believe is a cover for weapons development.

Brazil has also started carrying out uranium enrichment, and governments including South Korea, Japan, Saudi Arabia and Egypt are thought to be capable of and potentially interested in developing nuclear arsenals.

Only two countries have voluntarily reversed nuclear weapons programmes: South Africa, which gave up its fully-developed arsenal in the early 1990s, and Libya, which revealed and dismantled a programme in its early stages in 2003.