North Korea is one of the world's foremost providers of missile technology, but experts say there is still no evidence that Pyongyang has shared its suspected stocks of nuclear materials or chemical and biological weapons.

Arms experts around the globe say North Korea's military has long been equipped with chemical and biological weapons and is known as one of the biggest players in the missile trade.

Researchers in the United States and South Korea say the Stalinist state has sold missile technology and several hundred missiles to other countries for two decades. Most of the know-how came from one-time communist ally, the former Soviet Union.

These sales include SCUD missiles, with a range of about 500 kilometers and North Korea's Nodong system, which can travel more than 1000 kilometers. While North Korea has longer-range missiles, experts say it does not appear to have sold them.

Terry Taylor studies weapons proliferation in Washington for the International Institute for Strategic Studies, an independent research group. He says Pyongyang trades with other so-called pariah countries, such as Iran, Libya and Syria.

"It's significant as to who they're [North Korea] supplying it to [countries] who may be under some partial or complete sanctions, so they're the recourse for countries that have difficulty getting technology like this from anywhere else," said Mr. Taylor.

Furthermore, North Korea is one of the few nations suspected of having chemical weapons ready for use, such as sarin and mustard gas. Pyongyang also is believed to be developing biological weapons, agents that spread diseases such as anthrax and typhoid.

But experts say there is no evidence that North Korea has tried to profit from these weapons.

At the independent Carnegie Endowment for International Peace in the United States, proliferation expert Jon Wolfsthal says the world market for such weapons is limited.

"You'd have to export a lot of these things to make a significant difference, that's something that would be highly observable, and again, something that countries themselves might be able to accomplish without having to rely on North Korea," he noted.

A far bigger concern is North Korea's nuclear weapons programs.  U.S. intelligence agencies say Pyongyang probably has at least two crude nuclear weapons, and it claims to have reprocessed plutonium to make as many as eight.

At least some of Pyongyang's nuclear technology came from Pakistan

North Korea's nuclear weapons are considered a threat not only because Pyongyang might use them in a conflict, but also because they might be sold to terror groups or hostile nations.

The United States, South Korea, Japan, China and Russia have met three times since 2002 with North Korea to negotiate another nuclear disarmament deal, but have made no progress. Pyongyang, which has broken several international non-proliferation agreements, says it feels threatened by the United States, and wants security guarantees and economic aid in return for freezing its nuclear programs. Talks have stalled since September, when Pyongyang refused to attend a fourth round.

To keep countries such as North Korea from spreading weapons technology, more than a dozen nations have joined in Washington's 2003 Proliferation Security Initiative (PSI).  They are cooperating to track and stop ships suspected of carrying illicit weapons.

But some experts think the initiative has problems. They say the chief flaw is that it may not detect small weapons shipments, such as the eight kilograms of fissile material needed to make a crude nuclear weapon, or the workings of a missile guidance system.

That is why the current six-nation talks are seen as the best hope to get North Korea to dismantle its nuclear program and end any proliferation threat.

However, some experts fear efforts to stop North Korean missile proliferation could interfere with nuclear disarmament talks. Choi Choon-heum, a North Korea analyst at the Korea Institute of National Unification in Seoul, warns North Korea desperately needs the revenue from missile sales to keep the government afloat.

"They'll regard PSI as a signal to war.  They already said that, implementing PSI will be a signal to start a war," he added.

North Korea frequently threatens war in response to any efforts to make it comply with non-proliferation agreements.

Mr. Choi acknowledges that while missile sales are vital, nuclear sales would be another issue. He says Pyongyang realizes selling nuclear technology would incur the wrath not only of countries such as the United States, but also the North's chief ally, China.

Beijing provides up to half of the North's revenue and aid, and Pyongyang is not likely to risk losing that by expanding its weapons sales to include nuclear technology.