"We are intensely interested in this black market because it impacts on our ability to complete our work in Iran and Libya," International Atomic Energy Agency (IAEA) spokesman Mark Gwozdecky told AFP.
He was speaking after Pakistan President Pervez Musharraf Thursday rejected demands for an independent investigation, sharing of documents with the IAEA or opening of nuclear installations to UN inspections.
This followed revelations by the father of Pakistan's nuclear bomb, Abdul Qadeer Khan, that he had shared sensitive nuclear technology with Iran, Libya and North Korea for more than a decade.
"This is a sovereign country, no documents will be submitted to the IAEA, to an independent inquiry and we will not allow UN to supervise our nuclear" programme, Musharraf said.
But IAEA officials would be welcome to visit and Pakistan would discuss with them the results of its own investigation, he said.
Pakistan is a member of the IAEA but not a signatory to the nuclear Non-Proliferation Treaty, which empowers the agency to monitor worldwide compliance with nuclear safeguards.
"We have to look at the statute to see if there's the expectation that other member states would cooperate with the agency with its safeguards elsewhere," a Western diplomat at the IAEA said.
He said that if Pakistan was part of the global black market, "then it would behoove them to fix some of the damage they've done."
The revelations from Khan are just the "tip of an iceberg" about such illegal trafficking, IAEA chief Mohamed ElBaradei said Thursday.
"We need to follow this through. We need to know who was producing centrifuges" that can be used to make highly enriched uranium for atomic bombs.
He said Pakistan has been "quite cooperative so far" with the IAEA.
But a Western diplomat close to the IAEA said the agency had not asked Pakistan if it could interview Khan.
He said the political situation there was too sensitive. "The IAEA doesn't want Musharraf to fall. That would be the worst thing that could happen," he said.
The IAEA had set off the Khan scandal when it alerted Pakistan last year that Iran had blueprints for centrifuges that were similar to ones Pakistan had used in building the bomb and which Khan acquired when he worked in the Netherlands in the 1970s.
But a diplomat said nothing would have happened without US pressure on Pakistan to come clean on Khan.
"The United States put so much pressure on them. If it were just the IAEA, forget it, they couldn't do a thing," the diplomat said.
Analyst Jon Wolfsthal, who works in Washington's Carnegie Endowment for International Peace, said in Vienna: "The idea that this is the end of the story is impossible to accept."
ElBaradei had said individuals in at least five countries were involved in trafficking that went back at least to the 1980s.
The two countries besides North Korea, Libya and Iran were Malaysia and Dubai, Wolfsthal said.
Malaysia was a source of making parts for centrifuges. "They do manufacturing knock-off's quite well," said Wolfstahl about Malaysia's industrial capabilities.
Dubai on the other hand was a source for middlemen for the trafficking.
Wolfstahl said other countries involved might be Saudi Arabia, which in the 1980's bought long-range missiles from China, the Iraq of fallen dictator Saddam Hussein and maybe even Syria.
Saudi Arabia, said Wolfstahl, is concerned over Iran, which has been acquiring both nuclear and missile technology.
He said Syria was an unlikely candidate, however, for nuclear proliferation since it was mainly interested in chemical and biological weapons.
Meanwhile, "Khan probably did more with Iran than Iran has admitted," Wolfstahl said, referring to weapons design blueprints such as were found in Libya but not yet in Iran.
If such documents showed up, they could be the "smoking gun" for the IAEA to see Iran has failed to comply with international nuclear safeguards and take the issue to the UN Security Council, which could then impose punishing sanctions on Tehran.