North Korea's nuclear test blast in 2006, Iran's pursuit of potentially bomb-capable enrichment and new allegations Syria covertly tried to build an atomic reactor with North Korean help spotlight mounting challenges to the treaty, analysts say.

"The proliferation of nuclear weapons constitutes a threat to international peace and security," the United States, Russia, China, Britain and France said in a joint address on the final day of a two-week meeting of 106 NPT member nations.

"This ... imperils prospects for progress on other NPT goals such as nuclear disarmament and hurts prospects for expanding international (civil) nuclear cooperation," said British chief delegate John Duncan, speaking on behalf of the five.

"The proliferation risks presented by the Iranian nuclear programme continue to be a matter of ongoing serious concern to us." Tehran is under U.N. sanctions for refusing to suspend the work and curbing U.N. inspections meant to verify its nature.

Iran says it wants only electricity from enrichment, which can also produce atom bomb fuel if the process is adjusted.

The five said they stood squarely behind a revised packet of economic incentives which they, along with Germany, plan to present to Iran soon to shelve its enrichment programme.

The four Western powers and Russia and China have often struggled to agree on a mix of carrots and sticks for dealing with Iran. But they told NPT members they aimed to resolve the standoff with Iran "innovatively through negotiations".

They urged North Korea, which bolted from the NPT in 2003, to carry out a now-stalled six-party accord to disarm.

SYRIA ADDED TO NPT CONTROVERSY

Syria, like Iran repeatedly assailed by Washington and some allies at the meeting, was omitted from the powers' statement, `pparently due to reservations of some about U.S. intelligence purporting to show Damascus was close to completing a secret reactor before Israeli warplanes destroyed it last September.

Syria has rejected the findings as "forged" but is now under International Atomic Energy Agency (IAEA) investigation.

The five original nuclear arms powers spoke near the close of a taking-stock session by treaty states in which they again split along rich and poor lines over how to reinforce the NPT.

The five-power appeal aimed to bridge differences on ways to fix the NPT ahead of a decision-making review conference in 2010. The last one in 2005 collapsed in squabbling over agenda.

"It's the first time in eight years the P-5 (nuclear powers) have been able to make common ground on the way forward for the NPT, which has been damaged by our own disarray. Our step may look small but it's important," said a senior Western diplomat.

Nuclear states have said that what the NPT needs most is tougher safeguards on transfer of nuclear technology.

Developing states balk, saying this would wipe out their NPT right to peaceful uses of atomic energy. They also say the original nuclear powers have undermined respect for the treaty by clinging to arsenals as symbols of might and respect.

The five powers embraced IAEA-driven proposals for a multilateral nuclear fuel bank under depoliticised agency control to provide refined uranium to developing states as an alternative to indigenous development of enrichment.

This was a high priority for the 2010 conference, in addition to stronger non-proliferation safeguards, they said.