UN Secretary General Kofi Annan has called on world leaders to reinforce their commitment to a treaty aimed at reducing the threat from nuclear arms.
Many participants see open access to nuclear energy as a loophole
"We all bear a heavy responsibility to build an efficient, effective, and equitable system that reduces nuclear threats," he told a UN meeting.
He also called upon former Cold War rivals Russia and the US to reduce their current nuclear arsenals.
Mr Annan was addressing a conference on the Nuclear Non-Proliferation Treaty.
The treaty, entered into force in 1970, was designed to stop the spread of weapons, achieve nuclear disarmament and promote the peaceful use of nuclear energy.
It is reviewed every five years, with delegates from all 187 signatory states participating in the month-long conference at the UN's headquarters in New York.
'Under great stress'
Warning of the possibility of a nuclear catastrophe, Mr Annan said that "in our interconnected world, a threat to one is a threat to all, and we all share responsibility for each other's security".
"The plain fact is that the regime has not kept pace with the march of technology and globalisation, and developments of many kinds in recent years have placed it under great stress."
GLOBAL NUCLEAR POWERS
Signed the NPT: US, Russia, UK, France, China
Declared or known: India, Pakistan, Israel
Suspicions over: North Korea, Iran
Formerly had programmes: Algeria, Argentina, Belarus, Brazil, Kazakhstan, Iraq, Libya, Romania, South Africa, Ukraine
While countries such as Iran should be given the chance to reap the benefits of nuclear fuel, they must resist the temptation to pursue weapons programmes, the UN chief said.
Meanwhile, International Atomic Energy Authority head Mohamed ElBaradei urged Iran and the EU to keep talks on Tehran's nuclear programme alive.
"I would hope that the Iranians will not take any unilateral decisions to initiate any activities that now are currently suspended. I think that any future move has to be agreed between both parties," he said.
And, in an apparent attack on Iran and North Korea, the head of the US delegation said some states "continue to use the pretext of a peaceful nuclear programme to pursue the goal of developing nuclear weapons".
"We must confront this challenge in order to ensure that the treaty remains relevant," Assistant Secretary of State Stephen Rademaker said.
The issue of "dual use" equipment - technology which can be used to make both nuclear energy for peaceful purposes as well as bombs - is expected to feature prominently at the conference.
Many participants see the open access to the nuclear fuel cycle, and hence bomb fuel, as a loophole in the treaty that needs to be addressed.
For their part, many non-nuclear states criticise the failure of the five original nuclear powers - the US, Britain, France, China and Russia - to abandon nuclear weapons.
The non-nuclear nations say they are frustrated with the Bush administration's policies including the rejection of the nuclear test ban treaty, withdrawal from the anti-ballistic missile treaty and the development of new nuclear weapons.
The three other nuclear powers - India, Pakistan and Israel - have not signed the treaty.
Mr Annan himself emphasised that the only way to guarantee that no nuclear catastrophe takes place is to ensure that the world is free of such weapons.
"If we are truly committed to a nuclear weapon-free world, we must move beyond rhetorical flourish and political posturing, and start to think seriously how to get there," he said.
ESTIMATED NUCLEAR WARHEADS, STRATEGIC AND TACTICAL
*The US is also said to have some 3,000 warheads in reserve, while Russia has about 11,000 in non-operational stockpiles
Israel declines to confirm it has nuclear weapons
North Korea claims it has nuclear arms but no details are available
Iran is accused by the US of ambitions to build nuclear arms