UNITED NATIONS - After a 10-year gap, the United States on Thursday rejoined a biennial conference designed to win more support - including from the U.S. Senate - for the treaty banning all nuclear bomb tests.

The session brought together foreign ministers and other envoys from more than 100 nations that have ratified or at least signed the 1996 treaty. Secretary of State Hillary Rodham Clinton's participation represented the first U.S. official involvement since 1999.

The Comprehensive Test Ban Treaty, or CTBT, has lingered in a diplomatic limbo since a Republican-dominated Senate rejected it that year, but U.S. President Barack Obama has pledged to now "aggressively" pursue ratification.

Opening Thursday's meeting, U.N. Secretary-General Ban Ki-moon paid tribute to the American leadership.

"The participation of the United States led by Secretary of State Clinton for the first time demonstrates the commitment of the United States to work toward its ratification of the treaty," he said.

The two-day conference was being held in parallel with a summit of the 15 U.N. Security Council members on the subject of nuclear nonproliferation, presided over by Obama, who has pledged to work toward a world free of nuclear weapons.

"There is no better way to begin this historic day than to pledge to end nuclear testing," Ban told the assembled ministers. "The CTBT is a fundamental building block for a world free of nuclear weapons."

The conference adopted a declaration saying the treaty's entry into force is "more urgent today than ever before."

"We call upon all states which have not yet done so, to sign and ratify the treaty without delay, in particular, those states" - which includes the U.S. - "whose ratification is needed for entry into force," it said.

Thursday was the 13th anniversary of the ceremonial signing of the treaty by Mrs. Clinton's husband, then-President Bill Clinton, and other global leaders.

It was turned down in the Senate three years later when opponents objected that the U.S. might need to test its weapons to ensure the reliability of its nuclear stockpile, and contended that a planned International Monitoring System might fail to detect secret tests by nuclear cheaters.

Tibor Toth, who heads the U.N.-affiliated Comprehensive Test Ban Treaty Organization, said that this time Senate skeptics will have to confront the "reality" of a working, $1 billion verification network.

"I could call it a Verification Manhattan Project," Toth told The Associated Press, referring to the all-out U.S. program that built the first bombs in the 1940s.

Experts of the U.S. National Academy of Sciences are studying the effectiveness of the verification system, along with the reliability of the U.S. nuclear stockpile without testing, and will report their findings this winter.

Toth said he hoped this "nonpartisan" review will reassure enough Republicans to win the needed two-thirds ratification vote in the Senate, which now has a Democratic majority. Consideration is not expected until next year.

The pact requires ratification - that is, full government approval - by 44 nuclear-capable states before it can take effect. All but nine of those have ratified, along with the governing bodies of 115 other nations.

Besides the U.S., the holdouts among the 44 are China, Egypt, India, Indonesia, Iran, Israel, North Korea and Pakistan. Meanwhile, the U.S. and four other original nuclear powers - Russia, Britain, France and China - have observed testing moratoriums.

Indonesia has said it will ratify if the U.S. does, and analysts believe the Chinese would also follow suit. Most believe North Korea and Iran might be the final holdouts, and would be more deeply isolated internationally as a result.