By Jonathan Marcus BBC Diplomatic Correspondent

Is N Korea, a NPT signatory, developing nuclear weapons?

The entry into force of the Nuclear Non-Proliferation Treaty or NPT in March 1970 represented a milestone for a world living in the shadow of the nuclear bomb.

Its aim was to prevent the spread of nuclear weapons beyond the five "declared" nuclear armed powers: Britain, France, China, the United States and the then Soviet Union.

There were fears that without such an agreement there might be 15 or 20 nuclear-armed states within a similar number of years.

In that goal the Non-Proliferation Treaty has been successful.

But today, with countries like India, Israel and Pakistan effectively nuclear powers, and with growing worries over North Korea and Iran's nuclear activities, many wonder if the Non-Proliferation Treaty still serves a useful purpose.

Strength in numbers

Rose Gottemoeller of the Carnegie Endowment in Washington DC - herself a former senior US arms control official - argues emphatically that it does.

There are only three countries in the world, she told me, that are not members of the NPT regime - India, Pakistan and Israel.

Otherwise, it is almost a universally held treaty, so, she argues, "it is very important to remember that most countries of the world do live up to the regime and most countries do believe it is important".

The NPT regime's foundations, she insisted, remained firm.

The foundations may be strong, but many people fear that the super-structure is looking shakier than ever.

Weaknesses

Gary Samore is a non-proliferation expert at the International Institute for Strategic Studies in London.

He argues that a whole series of episodes had highlighted weaknesses in the regime - North Korea's withdrawal from the NPT after it was found to be cheating on its commitments, not to mention what he described as Iran's efforts to develop a nuclear weapons capability under the cover of a peaceful nuclear programme.

None of the nuclear weapon states are prepared to give up their nuclear arsenals Gary Samore Non-proliferation expert

In addition there was the Abdul Qadeer Khan's network in Pakistan, which demonstrated weaknesses in the whole export control system relating to nuclear technologies.

All of these events, he told me, had prompted a range of ideas to reform and strengthen the NPT regime.

This Review Conference will provide an opportunity for those different ideas to be discussed and debated.

But he doubts if any real consensus will be achieved.

Grand bargain

That is because the essential bargain at the heart of the NPT is under strain like never before.

The treaty: Q &A

At the heart of the Non-Proliferation Treaty's success lies a "grand bargain".

Other than the big five nuclear powers, all other countries joined the treaty as non-nuclear armed states.

They gave up any ambition to develop nuclear weapons; they agreed to open up all their facilities to inspection; and in return they were guaranteed the benefits of peaceful nuclear technology.

But, as Rose Gottemoeller told me, this 'bargain' is wearing a little thin and for good reason.

As she explained, "There is a very close relationship between the peaceful uses of nuclear technology, for energy purposes for example, and the creation of fissile material for nuclear bombs."

There was, she said, "great concern about countries like North Korea that can step up to the edge of the treaty constraints and then jump outside it" and this conference will be focused on that issue.

Worries on Iran

The worries over Iran's nuclear programme are very similar.

Could it gain a nuclear capability within the NPT regime and then simply abandon the treaty and press ahead with a weapons programme?

Indeed in the on-going talks between Iran and a trio of European countries, Tehran is actually being asked to give up any idea of having a nuclear fuel enrichment programme, something it is entirely within its rights to pursue under the NPT regime.

The Non-Proliferation Treaty in fact faces a crisis of compliance. Indeed it is a double crisis.

Because, as part of the "grand bargain", the five declared nuclear powers undertook eventually to give up their nuclear arms.

And many believe they have simply not been honouring this commitment.

Complex problems

As Gary Samore told me, "none of the nuclear weapon states are prepared to give up their nuclear arsenals" and each of them in some ways, perhaps with the exception of the UK, are actually taking measures to prolong their capability or even to find new roles for nuclear bombs.

HAVE YOUR SAY The Nuclear Non-Proliferation Treaty certainly still serves a purpose Josh, Traverse City, Michigan, USA Send us your comments

The US is investigating new types of nuclear weapons to attack deeply-buried targets; China is busy modernising its nuclear arsenal to make it more mobile; and Russia and China have both altered their nuclear doctrines in ways which might make the use of nuclear arms more likely.

"Their collective behaviour", he told me, "will certainly hang as a cloud over the Review Conference."

Few of the experts that I've spoken to had any great hopes that this gathering in New York would produce any ground-breaking initiatives.

Nonetheless the NPT still provides a basic bench-mark in a troubled world.

Its near universality is a great strength.

But there are doubts that the NPT is really sufficient to deal with the much more complex problems of non-proliferation in today's world.

Trying to re-draft the treaty would probably lead to it coming apart at the seams.

Many experts believe that real progress in dealing with the difficult cases like North Korea and Iran can only come from tailor-made diplomatic initiatives outside the treaty itself.