The dissident National Council of Resistance of Iran (NCRI) exposed Iran's uranium enrichment plant at Natanz and a heavy water facility at Arak in 2002. Analysts say it has a mixed record and a clear political agenda.
Iran says allegations that is trying to develop a nuclear weapons capability are baseless and forged by its enemies.
Iran and six world powers are trying to meet a self-imposed June 30 deadline to reach a comprehensive deal restricting its nuclear work. Issues remaining include monitoring and verification measures to ensure it cannot pursue a clandestine nuclear weapons program.
Citing information from sources inside Iran, including within Iran's Revolutionary Guards Corps, the Paris-based NCRI said a seven-person North Korean Defense Ministry team was in Iran for the last week of April. This was the third time in 2015 that North Koreans had been to Iran and a nine-person delegation was due to return in June, it said.
"The delegates included nuclear experts, nuclear warhead experts and experts in various elements of ballistic missiles including guidance systems," the NCRI said.
The Iranian embassy in France dismissed the report.
"Such fabricated reports are being published as we get closer to final stages of the talks and also because there is a high chance of reaching a final deal," Iran's state website IRIB quoted an unnamed Paris-based Iranian diplomat as saying.
There have previously been unconfirmed reports of cooperation between the two countries on ballistic missiles, but nothing specific in the nuclear field.
The UN Panel of Experts which monitors compliance with sanctions on North Korea has reported in the past that Pyongyang and Tehran have regularly exchanged ballistic missile technology in violation of UN sanctions.
SecrecyThe NCRI said the North Korean delegation was taken secretly to the Imam Khomenei complex, a site east of Tehran controlled by the Defense Ministry. It gave detailed accounts of locations and who the officials met.
It said the delegation dealt with the Center for Research and Design of New Aerospace Technology, a unit of nuclear weaponization research, and a planning centre called the Organization of Defensive Innovation and Research, which is under US sanctions.
Reuters could not independently verify the allegations.
"Tehran has shown no interest in giving up its drive to nuclear weapons. The weaponization program is continuing and they have not slowed down the process," NCRI spokesman Shahin Gobadi said.
UN watchdog the IAEA, which for years has been investigating alleged nuclear arms research by Tehran, declined to comment. North Korean officials were not available for comment.
Several Western officials said they were not aware of a North Korean delegation travelling to Iran recently.
A Western diplomat said there had been proven military cooperation between Iran and North Korea in the past.
"Import and export would be one of the key areas to monitor under any agreement with Iran," the diplomat said.
North Korean and Iranian officials meet regularly in general diplomatic activity. On April 23, Kim Yong Nam, North Korea's ceremonial head of state, and Iranian President Hassan Rouhani held a rare meeting on the sidelines of the Asian-African summit in Jakarta.