The top US government official in charge of arms control has resigned at a time when Washington is stepping up efforts to end North Korea's nuclear weapons drive and rallying global action over Iran's sensitive atomic program. Robert Joseph, the undersecretary of state for arms control and international security, sent a letter to President George W. Bush on Wednesday informing of his decision after six years on the job, the State Department said Thursday.

Considered a hardliner in the Bush administration, Joseph was a key architect of the controversial Proliferation Security Initiative (PSI) designed by the United States to combat the smuggling of weapons of mass destruction.

He is among at least half a dozen of senior officials from the State Department who had quit in recent months and nearly all of the jobs had not been filled.

Deputy Secretary of State Robert Zoellick resigned in July but his successor, the US intelligence chief John Negroponte, was named only this month.

Negroponte's confirmation hearing at the Senate is expected next week and more nominations to fill other positions are expected in the coming weeks, State Department spokesman Sean McCormack said, blaming the delay in filling the posts on bureaucracy.

"We are moving forward in filling each of the other remaining positions," he said.

"The personnel process grinds forward relatively slowly. That's certainly not a slight on people involved in that process, because there are reasons why this process is very careful and methodical. It's the right way to do things," he said.

earlier related report

UN nuclear agency warns of illicit nuclear material

Vienna (AFP) Jan 25 – The UN nuclear agency warned Thursday that the case of a Russian man who allegedly tried to sell a small amount of weapons-grade uranium could be a sign of wider availability of such dangerous material. The New York Times reported Wednesday that Georgia had sentenced a Russian man to eight and a half years in prison for trying to sell 100 grams of highly enriched uranium (HEU) in January 2005.

The International Atomic Energy Agency "is aware of the case and expects formal notification from Georgian authorities soon," IAEA spokeswoman Melissa Fleming said.

"Given the serious consequences of the detonation of an improvised nuclear explosive device, even small numbers of incidents involving HEU or plutonium (the two main explosive materials for atom bombs) are of very high concern," Fleming said.

Both the United States and Russia have worked in recent years to convert nuclear reactors that use HEU to run on low enriched uranium that is less of a proliferation risk.

The two main nuclear powers have also recycled HEU from nuclear weapons that are being retired.

Fleming said however that "trafficking incidents involving nuclear material point to possible weaknesses and may be indicative of the illicit availability of larger undetected quantities."

The IAEA has a data bank that lists 16 cases of incidents involving HEU or plutonium that could have been smuggled or was reported lost from 1993 to 2005.

In one case from 1993, 4.4 tons of beryllium including 140 kilgrams contaminated with some 150 grams of HEU "were discovered in the storage area of a bank" in the Lithuanian capital Vilnius, according to the illicit trafficking data.

In a second case in 1994, a total of 2.972 kilograms of HEU was found in St. Petersburg, Russia, when "an invidual was arrested in possession of HEU, which he had previously stolen from a nuclear facility.

"The material was intended for illegal sale," the data base said.

But these amounts of HEU are relatively small. A total of from 15 to 25 kilograms of HEU is normally needed to make one atomic bomb, although very high-tech weapons can use less HEU, according to experts.

But the Times said the Georgia case, which is not listed on the IAEA data base, "has alarmed officials because they had thought that new security precautions had tamped down the nuclear black market that developed in the 1990s after the Soviet Union collapsed."