UNITED NATIONS - A U.N. conference on Tuesday formally exhorted all nations to ratify a global ban on nuclear testing, but a U.S. boycott severely undercut the appeal and its likely success.

Nevertheless, organizers predicted Washington would eventually change its mind and make it possible for the Comprehensive Test Ban Treaty to take effect.

"I think if you keep up the pressure on the United States, they will come round," said Miguel Marin Bosch, a Mexican diplomat and conference president.

In a resolution approved after two days of speeches, the conference expressed concern that the pact, known as CTBT, had not entered into force five years after it was approved.

It urged countries that have not ratified the pact to do so and called on the United States -- the leading nuclear weapons state -- and other states with lesser nuclear capabilities to continue a voluntary moratorium in the meantime.

Despite a U.S. boycott, support for the treaty demonstrated by the conference was "very significant," insisted Olga Pellicer of Mexico, another conference official.

Participants decreed the treaty an "essential part" of the international non-proliferation regime. And while the voluntary testing moratorium observed by the United States and other nuclear weapons states is important, "it is not enough," she said.

Strong pressure on the United States came from Russia, the other major nuclear power, and the European Union.

Russia challenged U.S. objections and said disrupting the CTBT could lead to crisis and the "uncontained spread of nuclear weapons."

Moscow dismissed U.S. concerns that the pact would threaten the safety of U.S. and Russian nuclear arsenals and offered to work on new verification measures beyond treaty requirements.

The treaty, banning all nuclear blasts in the atmosphere, in space and underground, has been signed by 161 states. Of those, 87 have ratified it.

But the pact has not taken effect because it must be ratified by 44 specific states deemed nuclear arms-capable.

To date, 31 of those 44 countries including nuclear powers France, Russia and Britain have signed and ratified the pact.

Of the rest, India, Pakistan and North Korea have neither signed nor ratified the treaty while the United States, China and eight others have signed but not ratified.

Former U.S. President Bill Clinton, a Democrat, was the first world leader to sign the CTBT. But the Senate, then under Republican control, rejected it during the 2000 election.

Even before taking office, President George W. Bush, a Republican, made clear his strong opposition to the pact.

His administration has not publicly explained its decision to boycott the conference. But the Pentagon, hoping to hasten the treaty's death, for months argued privately in favor of the government sitting out the meeting.

Hard-liners wanted to go even further and have the United States take steps to cancel its signature.

Some administration officials believe the United States may have a need to test nuclear weapons in the future.

But Bosch said he has not given up hope the United States will someday ratify the treaty.

He noted that opinion in both the Bush administration and the U.S. Senate was divided on the CTBT, a dynamic that could eventually shift in the pact's direction.

Bosch said history was full of other seemingly lost causes in arms control, like the initial refusal of China and France to join the Nuclear Non-Proliferation Treaty. Both are now members.

"These things have a way of weighing on the souls of countries," he said. Nampa-Reuters