Concern over nuclear proliferation has risen following last month's confession by Pakistani scientist Abdul Qadeer Khan that he sold nuclear weapons plans and materials abroad.

The scale of this nuclear black market worries many observers, including experts at the United Nations' International Atomic Energy Agency, or IAEA, which serves as a watchdog against the spread of nuclear weapons.

While Libya, Iran and North Korea have already been named in the scandal, Melissa Fleming, head of public affairs for the IAEA, says the agency is concerned that others, including terrorist groups, also may have purchased weapons technology. She says Dr. Khan's apparent sale of bomb specifications is especially troubling. "One of the most disturbing signs in Libya was blueprints for nuclear weapons," says Ms. Fleming "And should a non-state actor, a terrorist group, get their hands on that, and then somehow through a black market, also get ready-made nuclear material - this could be very dangerous."

Even without ready-made plans, building a nuclear bomb is within the reach of any group with enough time and money. Pakistani nuclear physicist Pervez Hoodbhoy says the science behind bomb building is no longer the secret it once was. "The physics of that is now fairly easy. It requires maybe a graduate student. This could be his PhD project," he says. "And then to put together the whole thing, it needs engineers, technologists - maybe a group of 15 or 20 would be fine for this. And it needs something like a year or two."

The main difficulty for terrorists seeking a nuclear weapon would involve finding bomb-grade uranium or plutonium. Fortunately, neither of those materials occurs naturally, and experts agree that manufacturing either would require a major industrial base that even some nations would find impossible.

But Dr. Hoodbhoy notes that plutonium and enriched uranium can be obtained either as a by-product of nuclear power plants or from scrapped nuclear missiles. He says the nations of the former Soviet Union are among the likely sources of bomb material. "I think that's a cause for genuine worry, because . those stocks exist in Russia and in ex-Soviet Union countries where there are many hundreds if not thousands of nuclear weapons that have been dumped," says Dr. Hoodbhoy.

How well Russia and other nuclear states are faring at keeping such material out of the wrong hands is hard to know for sure, as the IAEA's Ms. Fleming explains. "The good news is that cases in trafficking in nuclear material have been few and far between, and the last major case, was several years back," she says. "So that could be a sign of either we're not detecting any kind of trafficking or, more hopefully, that nuclear material is as it should be: very well secured."

In an effort to keep bomb material safe, the IAEA works with U.N. members to track suspected cases of proliferation. Ms. Fleming says this cooperation is very strong, but adds that there is still room for improvement. "While we do get some very information from intelligence agencies, we consistently let them know that we can be more effective the better and the quicker the information we get."

The international effort to safeguard weapons materials is not the only factor limiting the chance of a nuclear terror attack.

Environmental physicist Fred Singer says terror groups would find it very difficult to build a weapon small enough to smuggle easily into a target nation. "If you want to make a bomb that fits into a suitcase, you need to have a fairly high efficiency," he says. "That's more difficult than assembling a bomb that you could put into a truck."