"Support and coordination from states would be needed for terrorists to produce WMDs," he said, speaking on Thursday at the WMD Terrorism: How Scared Should We Be? conference.

Blix acknowledged there was a small, but not zero, risk of terrorists laying their hands on WMDs, and he called for more preventive measures.

"Material and technology are now widespread, and an ability to create WMDs is also greater," he said.

Exaggeration

John Parachini, a political analyst with the California-based Rand Corporation, said the current threat of terrorists gaining such weapons had perhaps been exaggerated.

"WMDs are not easy to produce," he said, adding that "the mix of terrorism and WMDs becomes really dangerous if a group or groups form a sort of connection with a state and get knowledge from states how to produce WMDs".

"WMDs used by al-Qaida is much further off than we think," said Thomas Sanderson of the Strategic and International Studies' Transnational Threats Project.

He said the attacks on the World Trade Center in 1993 and 2001 showed that "the intention of terrorist groups to cause major damage is there".

"You don't need to kill thousands of people in order to cause a terrible effect on a country, as anthrax showed," he added, referring to a scare soon after the 2001 attacks.

According to Parachini, there have been four known cases in recent history of non-state actors using non-conventional weapons to wreak havoc: the Rajneeshee sect's salmonella poisoning of an Oregon town in 1984, the chlorine attack on the Sri Lankan air force carried out by the Tamil Tigers in 1990, the Aum sect's release of deadly sarin gas on a Tokyo subway train in 1995 and the deadly anthrax letters, thought to be of domestic origin, that terrorised the United States in 2001