International Atomic Energy Agency (IAEA) director general Mohamed ElBaradei told reporters on a flight from Vienna to Washington that he had "been in touch with Pakistan."
ElBaradei is to meet Wednesday with US President George W. Bush.
Pakistan has "been cooperating, but I still need more cooperation" from them in allowing "environmental sampling" to compare centrifuge components of a type sold through an international black market to Iran, ElBaradei said.
Iran claims contamination from particles on the imported components was the source of highly enriched uranium (HEU) discovered by the IAEA.
HEU can be used both as nuclear fuel in civilian reactors or as the raw material for an atomic bomb.
IAEA inspectors have found traces of HEU at two sites in Iran. The United States says the particles are proof that Iran is trying to develop nuclear weapons, despite Iran's claims of contamination.
The IAEA said in February that its investigation into Pakistani-led black market nuclear trading was on track, despite Islamabad's refusal to reveal documents or allow inspections of its facilities.
"We are intensely interested in this black market because it impacts on our ability to complete our work in Iran and Libya," IAEA spokesman Mark Gwozdecky told AFP on February 6.
He was speaking after Pakistan President Pervez Musharraf rejected demands for an independent investigation, sharing of documents with the IAEA, or opening of nuclear installations to UN inspections.
This followed revelations by Abdul Qadeer Khan, the father of Pakistan's nuclear bomb, 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.
The IAEA 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. Khan acquired those blueprints when he worked in the Netherlands in the 1970s.
ElBaradei said the Pakistani ambassador to the IAEA had said at an IAEA board of governors meeting in Vienna last week that the Pakistani government is "still investigating Mr. Kahn, even yesterday, even tomorrow."
"So I think I don't need to speak to him directly.
"We need to get all information from Paksitan through the Pakistani authorities. That's good enough for us," ElBaradei said.