An exiled Iranian opposition group accused Tehran Thursday of a "vast collaboration" with North Korea in developing nuclear arms, alleging that experts from both countries made regular intelligence-sharing visits.
"The Iranian regime continues to collaborate with North Korea on nuclear warheads and ballistic missiles," the National Council of Resistance of Iran (NCRI) said in a report citing sources close to the Iranian government.
North Korean experts spent a week in the Iranian capital in April this year, the report said, staying at a site close to the country's defence ministry.
It was the third such visit by a North Korean nuclear delegation in 2015 alone, according to the report.
Senior Iranian scientist Mohsen Fakhrizadeh was meanwhile in North Korea when it held a third nuclear test in February 2013, the NCRI said, and Iranian experts went to the country on a regular basis.
Impoverished but nuclear-armed North Korea is heavily sanctioned following a series of nuclear and missile tests staged in violation of UN resolutions.
Tehran has always denied seeking to develop nuclear arms, saying its nuclear programme is for civilian purposes only.
But the opposition group said Iran had "no intention" of renouncing what it said was an active strategy to acquire a nuclear bomb.
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The NCRI is a political umbrella of five Iranian opposition groups, the largest of which is the People's Mujahedeen Organization of Iran, which was once banned in Europe and the United States as a terror group.
US State Department spokesman Jeff Rathke said the report was unlikely to alter the progress of ongoing negotiations with Iran on curtailing its nuclear programme.
"We're examining the report. But we don't have any information at this time that would lead us to believe that these allegations impact our ongoing negotiations over Iran's nuclear programme.
"We have not been able to verify (the allegations) thus far," he said.
The People's Mujahedeen has long opposed the nuclear negotiations, and with the NCRI has made several important revelations of the existence of secret nuclear sites in Iran.
Iran on Thursday warned global powers against making "excessive demands" in talks aimed at sealing a deal, after France demanded access to its military installations.
US Secretary of State John Kerry will once again meet Iranian counterpart Mohammad Javad Zarif in Geneva on Saturday, after weeks of behind-the-scenes technical discussions in Vienna seeking to narrow the gaps on curtailing Iran's nuclear programme.
June 30 is the deadline for a comprehensive agreement.