Pakistan has sent parts of its nuclear centrifuges to the UN's International Atomic Energy Agency (IAEA) in Vienna.
Critics ask why fuel-rich Iran needs nuclear energy
The components are to be examined as part of an investigation by the UN nuclear watchdog into whether Iran has been trying to building an atomic bomb.
The IAEA wants to check whether traces of enriched uranium found at an Iranian nuclear site were illegally supplied by Pakistan's Dr Abdul Qadeer Khan.
He has admitted involvement in the transfer of nuclear secrets to Iran.
A spokesman for the IAEA in Vienna, Mark Gwozdecky, told the BBC the investigation was not against Pakistan's nuclear programme, but only to establish whether or not Iran had an active uranium enrichment programme.
Mr Gwozdecky described it as a highly sensitive matter and said all he could confirm was that a team of Pakistani scientists has arrived in Vienna with some samples.
The spokesman said the testing and analysis of the samples brought by the Pakistani team was already under way.
The IAEA has been investigating Iran's nuclear programme for the past two years.
While it has found no proof that Iran plans to build nuclear weapons, it has also been unable to confirm that the programme is entirely peaceful, as Iran insists.
Washington has accused Iran, a state already rich in gas and oil, of pursuing atomic energy as a screen to develop nuclear weapons.
In 2003, the IAEA found traces of uranium in Iran that had been enriched to various levels, some of them close to what would be useable in weapons.
Fears then arose that Iran had been secretly seeking to purify uranium for use in weapons.
Iran blamed the traces on contaminated centrifuge components it had acquired second-hand from Pakistan.