Japan, always nervous about China's nuclear power, said the test was "extremely regrettable", and had forced Tokyo to reconsider economic co- operation. It was the third Chinese nuclear test since the US, Russia, Britain and France declared a testing moratorium, and has raised concerns that France may resume its detonations.
Although China's action did not violate any firm international agreement, it was considered highly insensitive with the ink scarcely dry on last Friday's agreement to extend the treaty indefinitely. The NPT conference also agreed to a full ban on testing by the end of 1996,and detailed proposals for a Comprehensive Test Ban Treaty are being negotiated in Geneva.
China is attempting, before the ban begins, to complete a programme of miniaturising nuclear warheads so they can be mounted on submarine-launched ballistic missiles. It has done 42 tests, close to Britain's 44, suggesting a similar level of nuclear development.
Yesterday's explosion, several hundred metres below the desert in the Xinjiang-Uigur autonomous region in western China, measured about six on the Richter scale. It was estimated as being equivalent to 95,000 tons of TNT.
Nuclear testing is necessary for two reasons: to develop improved warheads or to verify that old warheads have not deteriorated. It is increasingly possible to use computer simulations, but the Chinese may be behind the US, Russia, Britain and France in computer technology. China, which has one submarine carrying 12 ballistic nuclear missiles, is endeavouring to develop this force to a level comparable with Britain and France.
Japan is concerned about its neighbours' nuclear potential. Apart from China, a declared nuclear weapons state, North Korea is believed to have a covert weapons programme. South Korea abandoned its programme in the late Seventies.