PARIS (REUTERS) - An exiled opposition group said on Thursday it had obtained information about a secret underground nuclear site under construction in Iran, without specifying what kind of atomic activity it believed would be carried out there.

The dissident National Council of Resistance of Iran (NCRI) in 2002 exposed Iran's uranium enrichment facility at Natanz and a heavy water facility at Arak. But analysts say it has a mixed track record and a clear political agenda.

In 2010, when the group said it had evidence of another new nuclear facility, west of the capital Tehran, US officials said they had known about the site for years and had no reason to believe it was nuclear.

The latest allegation comes less than a month after the election of a relative moderate, Hassan Rouhani, as Iran's new president boosted hopes for a resolution of the nuclear dispute with the West, and may be timed to cast doubt on any such optimism.

Iran says its nuclear programme is entirely peaceful and rejects accusations by the United States and Israel that it is seeking the capability to make nuclear weapons.

But its refusal to curb sensitive nuclear activity, and its lack of full openness with the UN nuclear watchdog, have drawn tough Western sanctions and a threat of pre-emptive military strikes by Israel.

The NCRI said members of its affiliated People's Mujahideen Organisation of Iran (PMOI) inside the country had "obtained reliable information on a new and completely secret site designated for (Iran's) nuclear project".

The NCRI which seeks an end to Muslim clerical rule in Iran, is the political wing of the PMOI, which fought alongside Saddam Hussein's forces in the Iran-Iraq war in the 1980s.

The NCRI said the site was located in a complex of tunnels beneath mountains 10km east of the town of Damavand, itself about 50 km north-east of Tehran. Construction of the site's first phase began in 2006 and was recently completed, it said.

The group released satellite photographs of what it said was the site. But the images did not appear to constitute hard evidence to support the assertion that it was a planned nuclear facility.