Some 150 countries meet here from Monday to review global efforts to check the spread of nuclear weapons, with the unresolved Iranian nuclear crisis looming large in the background.

Iranian President Mahmoud Ahmadinejad, whose country is likely to face fresh UN sanctions over its nuclear defiance, was expected to be among the first to speak at the opening session of the Nuclear Non-Proliferation Treaty (NPT) review conference.

But US Secretary of State Hillary Clinton, who will be among several foreign ministers attending, already warned him not to disrupt the parley, which is to kick off at 10:30 am (1430 GMT) Monday.

UN chief Ban Ki-moon, who is also to address the conference, has said that if Ahmadinejad "brings some good constructive proposal in resolving the Iranian nuclear issue, that would be helpful."

The review conference, which is held every five years, comes 40 years after the landmark NPT came into force and it aims to discuss how to further the treaty's full implementation and universality.

The focus in more than three weeks of discussions will be on the treaty's three main pillars: non-proliferation, disarmament and peaceful use of nuclear energy.

The NPT, the cornerstone of the global non-proliferation regime, is built on a grand bargain.

The five original nuclear weapon states -- Britain, China, France, Russia and the United States -- pledged to move towards disarmament. Non-weapon states forswore the bomb in return for access to peaceful nuclear energy.

But since the treaty came into force in 1970, Israel, India, Pakistan, and North Korea have all acquired a nuclear weapons capability. A total 190 countries have signed the treaty, including North Korea which however withdrew from it in 2003.

Israel, India and Pakistan have been invited at the conference, albeit without the right to speak, but none have said whether they will attend.

Iran and many nonaligned nations argue that the NPT is discriminatory, dividing the world into an elite of nuclear haves lording over second-class nuclear have-nots.

Thursday, the president-elect of this year's conference, ambassador Libran Cabactulan of the Philippines, said all the parties could bring their views to the table.

The meeting, he added, should serve as "a marketplace of ideas (where) the best ideas or the right ideas must prevail."

But some diplomats fear that the Iranian nuclear issue may prove a distraction.

Ahmadinejad, whose country has signed the NPT, is likely to use his presence to defend his country's right to master the full nuclear fuel cycle, including uranium enrichment.

The Iranians, backed by Arab countries, also want to turn the spotlight on Israel's sizable nuclear arsenal.

Tehran vehemently denies US charges that it is using his uranium enrichment program to mask a bid to acquire nuclear weapons.

The United States and its European allies had hoped to have new sanctions against Iran adopted by the full Security Council before the NPT conference.

But negotiations among six major powers -- Britain, China, France, Germany. Russia and the United States -- have been dragging on and a vote by the full council is unlikely before the end of the conference on May 28.

Key issues facing NPT signatories include how to make the treaty universal, how to prevent non-state actors from acquiring nuclear weapons, how to improve monitoring of nuclear programs and advance the peaceful and safe use of nuclear energy.

A major stumbling block may be Egypt's insistence, backed by non-aligned states, that Israel should join the NPT and its call for an international conference on creating a nuclear weapons free zone in the Middle East. The 1995 review conference had called for such a zone.

Israel is believed to have some 200 atom bombs but neither confirms nor denies this. It says it backs the idea of a nuclear weapons-free zone but insists there must first be a Middle East peace agreement.

Despite the hurdles, UN and US officials are voicing guarded optimism, citing a positive atmosphere created by a recent US-Russian successor accord to the 1991 Strategic Arms Reduction Treaty.

The new START cuts the number of deployed warheads by 30 percent from levels set in 2002, specifying limits of 1,550 nuclear warheads for each country.

But it will be hard to agree on a final document, let alone one with specific changes, such as universalizing the Additional Protocol that allows for tougher inspections of a country's nuclear activities, observers said.

The last of the five-year reviews failed in 2005 over these issues and did not even release a final document.