The US National Intelligence Estimate released last week said that Iran halted a nuclear weapons development program in 2003 because of international pressure. Mohaddessin told a news conference that Iran appeared to have duped US intelligence into that conclusion.

"The clerical regime leaks false information and intelligence to Western intelligence services, through double agents," he said.

Mohaddessin said Iran did shut down a Teheran weapons program center known as Lavizan-Shian in 2003 under international pressure and demolished the site. However, Mohaddessin claimed the Iranian authorities shifted their weapons program to other sites, which resumed the work in 2004.

The NCRI is the political wing of the People's Mujahedeen of Iran, an opposition group that advocates the overthrow of government in Teheran. The Mujahedeen have been designated a terrorist group by Iran and by both the United States and the European Union.

It was not possible to independently verify the NCRI claims, which Mohaddessin said came from sources within Iran, including some among staff at covert nuclear plants.

Four years ago, the group disclosed information about two hidden nuclear sites that helped uncover nearly two decades of covert Iranian atomic activity. But much of the information it has presented since then to back up claims that Iran has a secret weapons program has not been publicly verified.

Mohaddessin said Iran was continuing to develop nuclear weapons technology at a site near the original plant in the Teheran neighborhood of Lavizan and other units around the country. He said the group had checked with its sources in the past week and discovered that the centers were still working.

"These centers are working just now for producing nuclear bombs. This is contrary to the United States' National Intelligence Estimate," he said.

Iran claims its nuclear development is peaceful and aimed at producing energy.

Meanwhile, Iranian President Mahmoud Ahmadinejad on Tuesday called the report a "step forward" and said more such steps could create an "entirely different" situation between the two countries.

"We consider this measure by the US government a positive step. It is a step forward," Ahmadinejad told a press conference.

"If one or two other steps are taken, the issues we have in front of us will be entirely different and will lose their complexity, and the way will be open for the resolution of basic issues in the region and in dealings between the two sides," Ahmadinejad said.