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* Test ban treaty agreed in 1990s but still not global law
* U.S. and seven other states need to ratify pact
* U.S. Senate rejected treaty in 1999, new push planned
By Fredrik Dahl
VIENNA, Oct 23 (Reuters) - A 183-nation body set up to monitor a ban on nuclear bomb tests elected a new head on Tuesday to face the tricky task of helping convince the United States and other hold-outs to finally turn the landmark treaty into global law.
After four rounds of voting to separate the five candidates, Burkina Faso geophysicist Lassina Zerbo was picked as new executive secretary of the Preparatory Commission for the Comprehensive Nuclear Test Ban Treaty Organization (CTBTO).
Seen as a cornerstone of efforts to free the world of atomic bombs, the pact, negotiated in the 1990s, enjoys wide support around the world but still needs to be ratified by eight more so-called nuclear technology states to come into force.
Backers of the CTBT say it has already had a major impact in reducing the number of nuclear tests in the world but that this achievement may be jeopardised if it does not take legal force.
"The next two years are very critical," an African diplomat said, warning that some countries may "start losing interest".
Zerbo - who now heads the CTBTO's International Data Centre and will succeed Hungary's Tibor Toth - can play an important role in promoting the test ban by showing the effectiveness of the agency overseeing it, diplomats say.
The administration of U.S. President Barack Obama - who is up for re-election next month - has said it is preparing a new push for Senate approval, arguing the country no longer needs to conduct atomic tests but does need to stop others from doing so.
More than 2,000 nuclear tests were carried out between 1945 and 1996, when the CTBT opened for signature, most of them by the United States and the then-Soviet Union. Since then, only India, Pakistan and North Korea have conducted such blasts.
"But until the CTBT enters into force, the door to testing is still open," said Daryl Kimball of the Arms Control Association, a Washington-based advocacy and research group.
The United States is one of eight countries - together with China, India, Pakistan, Israel, Iran, North Korea and Egypt - whose ratification is needed for the pact that has so far been ratified by 157 states to take effect.
Proponents say U.S. ratification of the pact, rejected by Washington lawmakers in 1999, could encourage others to follow.
They say a global test ban would make it more difficult both for non-nuclear states to acquire atomic bombs and for nuclear powers to develop even more advanced weapons.
The United States and China are two of the world's five officially recognised nuclear weapons states, together with Britain, Russia and France.
India, Pakistan, North Korea and Israel are outside the separate nuclear Non-Proliferation Treaty, the 1970 pact meant to prevent the spread of nuclear arms. Iran is part of the NPT but the West accuses it of seeking covertly to develop a capability to build atomic bombs. Tehran denies the charge.
At the time of the U.S. Senate vote on the CTBT 13 years ago, opponents argued that a permanent end to testing could erode the reliability of the U.S. nuclear arsenal. The United States last carried out a nuclear test two decades ago.
The CTBTO has established a system to detect any nuclear blasts, with more than 280 monitoring sites in the world.
An adequately funded and effective "verification regime will hasten the treaty's entry into force by demonstrating the capability to detect and deter violations," U.S. diplomat John Godfrey told delegates at this week's CTBTO meeting in Vienna.
But nuclear expert Henry Sokolski said the treaty was unlikely to get past U.S. Republicans "still upset" about the New Strategic Arms Reduction Treaty (START) with Russia, which the administration had difficulty winning Senate backing for. (Editing by Mark Heinrich)