Finnish Foreign Minister Erkki Tuomioja, who is president of the conference taking place in the Austrian capital, said the move sent completely the wrong message to the international community.

"We should be concerned about the development of weapons of mass destruction, even in the case of low yield weapons," Tuomioja said in an interview to be published in the Austrian daily Die Presse on Friday.

He said their development "sends completely the wrong signal, which consists of saying: 'If you can, use your weapons of mass destruction as a deterrent.' I consider this approach extremely negative," he said.

The head of the International Atomic Energy Agency (IAEA) Mohamed ElBaradei accused the United States last week of effectively breaking a ban on the proliferation of weapons of mass destruction through its research into so-called "mini-nukes."

In an interview he told the German weekly Stern that double standards were being used.

"The US government insists that other countries do not possess nuclear weapons. On the other hand they are perfecting their own arsenal. I do not think that corresponds with the treaty they signed."

The United States has shunned the three-day meeting of the Comprehensive Test Ban Treaty's (CTBT) signatories.

In 1996 it became the first state to sign the CTBT, but the US Senate in 2000 refused to ratify the treaty and Washington has since indicated it has no intention of doing so.

In May, the US Congress lifted a ban on research into low-yield nuclear weapons, or "mini-nukes", meaning up to about one-third of the yield of the bomb that destroyed Hiroshima in World War II.

According to press reports the United States is alleged to have already carried out 18 so-called 'sub-critical' tests, which do not provoke a chain reaction.

Countries that sign the treaty agree not to conduct any kind of nuclear testing. So far, 168 nations have made that pledge, and 104 have obtained full ratification of the treaty from their national parliaments, including nuclear powers Britain, France and Russia.

Under the treaty rules, the 44 nations that had nuclear research or power facilities when it was adopted in 1996 must ratify it. Unless they do so, the agreement may lapse, diplomats have warned.

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