A senior Democratic arms-control advocate has said the Senate - dominated by Republicans - will not approve the treaty, which was launched three years ago.

The BBC's Samantha Simmonds: "President Clinton faces substantial opposition"

Delaware Democrat Senator Joseph Biden made his comments as the Senate Armed Services Committee opened three days of hearings on the treaty.

So far 154 nations have signed but the treaty will not go into effect until it has been ratified by a further 18 countries. The United States was the first to sign but the Senate has refused to ratify.

The United Nations deputy secreatary-general, Louise Frechette, has warned that further delay in bringing the treaty into force will increase the risk of nuclear testing being resumed.

Fighting talk from Clinton

President Clinton has promised an all-out fight to get the treaty through, and on Tuesday said that rejection would have implications for American leadership on the issue.

"It would say to the whole world: 'Look, America's not going to test but if you want to test, go right ahead. We're not interested in leading the world toward non-proliferation anymore.'"

BBC's Nick Bryant in Washington: "Treaty is centrepiece of Clinton's foreign policy"

Senate conservatives say the CTBT should be rejected because of verification problems.

Conservatives' fears

They believe it would not stop the nuclear ambitions of North Korea and Iran and would harm efforts to maintain the safety and reliability of the US nuclear arsenal.

The United States stopped testing in 1992 and President Clinton says it has no intention of resuming.

The United States is one of more than 154 signatories to the CTBT.

On Monday, in a move seen as an attempt to allay Senate concerns, the US Government confirmed it was trying to get better access to Russian nuclear test sites.

The Central Intelligence Agency has concluded that it was unable to accurately monitor low-level tests at a key site in the Russian Arctic.

Sen Trent Lott could cancel the vote

Sen Biden said that, while all 45 Senate Democrats were prepared to support the CTBT in a vote scheduled for next Tuesday, Republican support was vanishing.

At least 22 Republican senators would have to join the 45 Democrats to achieve the necessary two-thirds majority for the pact to be ratified.

Senate Majority leader Trent Lott said he would consider delaying next Tuesday's vote.