China and Iran have withdrawn their contribution to monitoring nuclear tests, apparently in protest at Washington's hostility towards the comprehensive test ban treaty, raising the fear that the treaty may collapse before it has come into operation.
Both have stopped sending seismic data to the nascent Comprehensive Test Ban Treaty Organization (CTBTO) in Vienna.
The treaty will only come into force when it has been ratified by the 44 states believed to be capable of building nuclear weapons. So far it has been ratified by 31.
The Bill Clinton administration signed the treaty, but three years later the US Senate refused to ratify it, and the George W. Bush White House has openly opposed it, hinting that it may end the moratorium on testing which has been in effect since 1992.
In its nuclear posture review, leaked last month, it ordered work to begin on developing a new generation of low-yield tactical nuclear weapons such as "bunker-busters," which would require testing before being used.
Diplomats in Washington said US opposition to the treaty was a major barrier to gathering the international support required to bring it to life and argued that the reduced cooperation of China and Iran could be signs of a backlash which could eventually kill it.
Iran has completed work its monitoring stations, among 337 around the world intended to send a stream of data to the CTBTO, allowing it to spot a nuclear test anywhere on earth. But the Iranian parliament has stopped the stations sending data until the treaty enters into force.
China has yet to complete testing its monitoring stations and has cut off a direct flow of data from other seismic stations, sending the information on computer diskette in a diplomatic pouch.
Beijing protested last week that the US nuclear posture review had "openly stated that nuclear weapons would be used" in the event of military conflict in the Taiwan Strait and breached an agreement on "mutual non-targeting of their nuclear weapons."