But nuclear states China and India said they would continue moves towards ratification.

Russia said that until its technology caught up with the US, it would not ratify the agreement.

The head of the Russian Parliament's defence committee, Roman Popkovich, said that the US had sufficient technology to simulate nuclear weapons tests, and Russia was postponing its ratification until this discrepancy was removed.

"Being able to set up certain types of test stands, [the US] has been able to continue the development of their weapons, experimentally, without nuclear tests and to take part in the creation of completely new programmes. We have not had this opportunity."

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Listen to President Clinton's statement

The US Senate voted against ratifying the CTBT on Wednesday.

A spokeswoman for the Chinese Foreign Ministry said Beijing "deeply regretted" the Senate decision, but would speed up its own efforts to ratify the treaty and continue its self-imposed moratorium on nuclear tests.

The newly-elected Indian Government said it would try to build domestic political support for the treaty.

India strongly opposed the treaty as it was being negotiated, but, after testing its own nuclear devices last year, started working towards accepting some of its conditions.

The foreign minister, Jaswant Singh, also said India's voluntary moratorium on further tests would remain in effect.

There has been no response to the US vote from Pakistan which also completed nuclear tests last year.

Global setback

The Senate's rejection of the CTBT is being seen as a blow to attempts to halt the spread of nuclear weapons.

The treaty has already been signed by more than 150 countries, but cannot come into force until it is ratified by all 44 countries judged to have nuclear capability.

Russia and China had been waiting for the US to ratify first.

US President Bill Clinton described the Senate Republicans who had voted against the treaty as "reckless" and "partisan".

It was the first time the Senate had rejected an arms-control pact.

The BBC's Rob Watson: "Democrats argued the treaty would prevent the spread of nuclear weapons"

It was also a major foreign policy defeat for President Clinton, who had fought for US endorsement of the accord.

He was the first world leader to sign the treaty in 1996 and has set nuclear non-proliferation as a major foreign policy goal.

Following the vote, President Clinton said his government would observe a moratorium.

Showdown

The Senate rejected the CTBT by a vote of 51 to 48. The final margin fell a long way short of the two-thirds majority - 67 votes - required to ratify the pact.

President Clinton's impassioned plea to Republicans fell on deaf ears

The vote closely followed party lines, with only four Republicans breaking ranks to endorse the treaty.

Michigan Democrat Senator Carl Levin said the vote had made the world "a more dangerous place".

Senate Majority Leader Trent Lott, a Mississippi Republican, had called the test ban treaty "fatally flawed".

New Nato Secretary-General, former UK Defence Secretary Lord Robertson, told the BBC that the Senate decision was "very worrying".