The 50 participants taking part in the closed-door talks ending Tuesday hope to exchange ideas before an international meeting in New York in May on the treaty.

"The treaty faces a grave challenge as we see problems in North Korea and Iran," Japan's vice foreign minister Hideyoshi Tanigawa said at the opening of the talks.

Iran has agreed with Britain, France and Germany to suspend all activities related to uranium enrichment in return for talks on trade, security and technological bonuses for the Islamic republic.

But the EU negotiators are demanding Iran totally dismantle its nuclear fuel program to guarantee it does not seek atomic weapons.

The United States has accused North Korea of operating a secret uranium-enriching program to produce weapons in breach of a 1994 agreement.

Pyongyang has indicated it may return to six-nation talks on the issue but no date has been set.

Speaking at an unrelated seminar in Tokyo Monday, John Bolton, US undersecretary of state for arms control and international security, said the non-proliferation treaty had serious problems over compliance.

"I think there is no doubt that the Nuclear Nonproliferation Treaty is in a period of crisis, a crisis of compliance, and a crisis caused largely by the efforts of states that are committed to be non-nuclear weapon states which are rejecting or violating commitments they made," he said.

Nuclear powers Britain, China, France and the United States are taking part in the talks.

However, declared nuclear weapons states Russia, India and Pakistan did not attend, said a Japanese foreign ministry official who did not have further details. Iran and North Korea were also absent.

Participants included Sergio de Queiroz Duarte, the chair of the New York meeting which will review progress on the Nuclear Nonproliferation Treaty.

Japan has championed the treaty as it is the only country to have suffered nuclear attack and is engaged in a heated row with North Korea.

UN atomic energy chief Mohamed ElBaradei has proposed that the New York meeting discuss a five-year international moratorium on building facilities for uranium enrichment and nuclear reprocessing, so that nuclear-armed states could lead by example.