At the dawn of the nuclear age, expertise in nuclear matters was limited to scientists of the two superpowers, the United States and the Soviet Union.  Now nuclear arms technology is much more accessible to governments or, potentially, terrorist groups.  That has greatly complicated efforts to halt the spread of nuclear weapons.

Earlier this year, it was revealed that Pakistan's top atomic scientist, A.Q. Khan, had been peddling nuclear materiel and know-how to governments harboring nuclear weapons ambitions.  Former chief U.S. weapons inspector David Kay says it was a shock to international attempts to curb the spread of nuclear weapons.

"We now know, or we think we know, that he was offering services and materials for about 20 years internationally before anyone had the goods on him and could prove that he was doing it and try to break up that network,? said Mr. Kay.  ?So, while it is good that the A.Q. Khan network, at least in part, has been shut down, it is really disturbing that it carried on for so long without being detected and shut down."

The legal cornerstone of non-proliferation efforts is the Nuclear Non-Proliferation Treaty.  The pact, with 188 signatories, binds non-nuclear states to refrain from seeking nuclear weapons.  In return, it calls on the declared nuclear powers to negotiate among themselves for the dismantling of their own nuclear arsenals.  Matthew Bunn, senior research associate of the "Managing the Atom" project at Harvard University, says the non-nuclear states in return can get assistance for peaceful nuclear technology.

?It sort of offers the carrot of civilian nuclear technology and a commitment by the weapons states to negotiate towards disarmament in return for states voluntarily giving up their right to have nuclear weapons," he said.

South Africa and most recently Libya have abandoned their nuclear weapons programs.  But North Korea has withdrawn from the pact.  Iran is accused of pursuing nuclear weapons in defiance of it.  Pakistan and its neighbor and rival India have both tested nuclear weapons and are among the few countries that have not signed the treaty.

Mr. Bunn and other nuclear experts say treaties alone are ineffective, especially if countries cannot agree on how to enforce their provisions.  The United States and Europe have fundamental disagreements over how to deal with Iran's pursuit of nuclear weapons technology, and a cohesive strategy for dealing with North Korea has proved elusive.

Spurred on by the A.Q. Khan case, President Bush earlier this year announced an expansion of the U.S. "Weapons Security Initiative," emphasizing the "carrot and stick" approach.

"Abandoning the pursuit of illegal weapons can lead to better relations with the United States, and other free nations,? said Mr. Bush.  ?Continuing to seek those weapons will not bring security or international prestige, but only political isolation, economic hardship, and other unwelcome consequences."

David Kay says the initiative's emphasis on information-sharing, especially in an age of heightened concern about terrorism, is the right approach.

"It is a commitment and a much greater sharing of information, and I suspect that is the prototype we will be seeing more of in the coming years, not necessarily treaty-based regimes, but the agreement of states that they have to do something and just getting on and doing it, rather than negotiating lengthy international treaties," he added.

When it comes to terrorists, say experts, the primary concern is over their possible acquisition of actual weapons - possibly from the former Soviet Union, or the materiel to make them.  Matthew Bunn says many nuclear facilities are poorly defended.

"There are, in some 40 countries around the world, civilian, and therefore not very heavily defended, research reactors that use highly enriched uranium as their fuel, which is the easiest material in the world for terrorists to use to make a nuclear bomb,? he explained.  ?There are more than 130 of these reactors still using highly enriched uranium as their fuel.  Many of these reactors are on university campuses or other completely indefensible locations.  Many of them have no more security than a night watchman and a chain link fence."

The United States has embarked on several programs to buy back nuclear fissile material such as highly enriched uranium, which can be used to make bombs, that has been dispersed around the world over the years, some of it originally from U.S. stockpiles.  But according to recent Energy Department audit, a considerable amount of weapons-grade material is still unaccounted for.

The United States offers assistance to enhance physical security at nuclear sites and to buy back old Soviet nuclear weapons.  Under legislation passed in 1991 just after the fall of the Soviet Union, the United States set up a fund to stabilize and secure Soviet nuclear stockpiles, and to destroy old weapons in Russia and other former Soviet states.

Mr. Kay says there have been instances of people offering nuclear materiel for sale out of the old Soviet Union.  Most, but not all, he says, have turned out to be frauds.

"We have found a number of cases.  Fortunately, a lot of them have been stings of people offering nuclear material.  It has been radioactive material, not totally harmless, but not likely to produce a bomb.  But there have been three or four cases of real plutonium, weapons-usable material, being offered from being smuggled out of the [former] Soviet Union," he said.

The Nuclear Non-Proliferation Treaty is reviewed at an international conference every five years, with the next review scheduled for 2005.