Pakistan has confirmed that the former head of its nuclear weapons programme, AQ Khan, gave centrifuges for enriching uranium to Iran.
Khan confessed last year to leaking nuclear secrets
It is the first time Pakistani officials have publicised details of what nuclear materials the disgraced scientist passed on to Iran.
Information minister Sheikh Rashid Ahmed told the BBC's Urdu service that "a few" centrifuges were involved.
Iran is under international pressure over its nuclear ambitions.
It says it intends to use enriched uranium only in power stations, but the US says Iran is making fuel for nuclear weapons.
The Pakistani information minister stated again on Thursday that his government had no knowledge of Dr Khan's activities.
"He helped Iran in his personal capacity," he said, the Associated Press news agency reported.
Officials have consistently said that the government had no knowledge of Mr Khan's activities on the nuclear black market.
Many analysts have questioned the veracity of these denials saying it would have been impossible for him to conceal his actions.
Dr Khan remains under close guard at his home in Islamabad.
The authorities have refused to allow experts from the UN nuclear watchdog, citing national security.
"We will not hand over [Dr Khan] to any other country," Mr Ahmed reiterated on Thursday.
Last month he dismissed reports that the US was probing whether Dr Khan had sold nuclear secrets to Arab nations.
European countries and the UN recently joined the US in criticising Iran for allegedly not keeping a pledge to suspend uranium enrichment activities.
UN atomic energy agency chief Mohamed ElBaradei said this month that the "ball is very much in Iran's court to come clean".
The US has called Dr Khan the "biggest proliferator" of nuclear technology.
Labelled the father of Pakistan's nuclear programme, Dr Khan confessed last year to leaking nuclear secrets.
He said he took full responsibility for proliferating nuclear weapons to Iran, Libya and North Korea.
President Pervez Musharraf pardoned him, but the scandal embarrassed and traumatised Pakistan, and stunned world nuclear experts.
Dr Khan had held the post of scientific adviser since retiring as head of the country's top nuclear facility in 2001 but was sacked after his confession.