Iran insists that it made a full declaration of its nuclear technology to the International Atomic Energy Agency (IAEA) in October, and that its programmes are purely peaceful.
Diplomats said on Thursday the parts that had been found were compatible with the "P2" uranium-enrichment centrifuge, which is a Pakistani version of the advanced Western "G2" design, and can produce material for use in reactors or bombs.
"This stuff should have been declared," one Western diplomat said. The IAEA declined to comment and Iranian officials could not immediately be contacted.
The daily USA Today reported that the parts had been found at the Doshen-Tappen air base and quoted one unnamed source as saying a system had been built and tested.
Iran said this week it had held blueprints for the G2 system, but denied allegations from Western diplomats that it had committed a serious offence by failing to declare them.
"(This) is not something the agency has discovered, Iran has informed the agency about it...It's a sheer lie that Iran is manufacturing G2 centrifuges"
Head, foreign relations, Iran's Supreme National Security Council
"(This) is not something the agency has discovered, Iran has informed the agency about it...It's a sheer lie that Iran is manufacturing G2 centrifuges," Husain Mousavian, head of foreign relations at Iran's Supreme National Security Council, said in a newspaper published on Monday.
IAEA chief Muhammad al-Baradai is expected to circulate two reports next week on UN inspections, one on Iran and the other on Libya, which admitted in December to pursuing weapons of mass destruction and agreed to give them up voluntarily.
Iran agreed in October, in a deal brokered by Germany, France and Britain, to allow snap inspections of its atomic facilities by the IAEA and to suspend uranium enrichment.
Iran agreed last year to allow
snap inspections of nuke facilities
The discovery of the G2 designs led some arms experts to speculate that Iran may have a secret enrichment facility apart from one at Natanz in the centre of the country, which is being built to accommodate older G1 centrifuges.
Several diplomats and arms experts have said they believe the Pakistani nuclear scientist Abd al-Qadir Khan, who has admitted leaking nuclear secrets to Iran, Libya and North Korea, offered Iran his centrifuge designs on the black market.
Gas centrifuges spin at supersonic speeds to separate fissile uranium-235 from the non-fissile uranium isotopes.