An exiled Iranian opposition group said on Thursday that a delegation of North Korean experts in nuclear warheads and ballistic missiles had visited a military site near Tehran in April amid talks between world powers and Iran over its nuclear program.

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. But analysts say it has a mixed track record and a clear political agenda. Iran says allegations of nuclear bomb research 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, but issues remain including the monitoring and verification measures to ensure Iran could not pursue a clandestine nuclear weapons program.