The leader of an exiled Iranian opposition group addressed US lawmakers for the first time Wednesday and warned of the links between Shiite Iran and the Sunni Muslim Islamic State (IS) militants.

"It was the mullahs' regime who helped the creation of ISIS... and the killing of Sunnis in Iraq helped the emergence of ISIS," Maryam Rajavi told House lawmakers, using another acronym for the IS jihadist group that has captured a swathe of territory in Iraq and Syria.

Rajavi is the president-elect of the National Council of Resistance of Iran, a political umbrella coalition of five Iranian opposition groups that includes the once blacklisted People's Mujahedin of Iran (MEK).

The Paris-based group has proved controversial in the past due to its links with the MEK, formed in the 1960s to overthrow the then shah of Iran.

Once the shah went into exile, the MEK fought the emergence of the Islamic leaders in Tehran, earning itself a listing as a "terrorist organization" by the State Department in 1997. It was only removed from a US terror blacklist in 2012.

Although Iran has been helping Iraqi leaders to fight the Sunni IS militants by arming and advising Shiite militias, Rajavi insisted that Iranians called Iran's religious leaders "the godfather" of IS.

"It was the Iranian regime ... that created terrorism as a major threat for stability," Rajavi said, appearing at the House foreign affairs committee via a live satellite link from her Paris offices.

"Terrorism and fundamentalism under the name of Islam came to the world as seen by the mullahs' regime in Iran, and when this regime is overthrown, that will be limited or destroyed."

And she alleged that "despite all their differences, ISIS is very close to the fundamentalists ruling in Iran" and even on "occasions they have cooperated."

"The mullahs' regime is not part of any solution as we attempt to deal with Islamic fundamentalism. It is indeed the heart of the problem," she added, insisting the "ultimate solution" was regime change in Iran.