A former undercover informant says he provided evidence to the FBI during President Obama's first term that Russia was assisting Iran's nuclear program even as billions in new U.S. business flowed to Moscow's uranium industry.

William Douglas Campbell told The Hill his evidence included that Russia was intercepting nonpublic copies of international inspection reports on Tehran's nuclear program and sending equipment, advice and materials to a nuclear facility inside Iran.

Campbell said Russian nuclear executives were extremely concerned that Moscow's ongoing assistance to Iran might boomerang on them just as they were winning billions of dollars in new nuclear fuel contracts inside the United States.

"The people I was working with had been briefed by Moscow to keep a very low profile regarding Moscow's work with Tehran," Campbell said in an interview. "Moscow was supplying equipment, nuclear equipment, nuclear services to Iran. And Moscow, specifically the leadership in Moscow, were concerned that it would offset the strategy they had here in the United States if the United States understood the close relationship between Moscow and Tehran."

A spokesman for former President Obama did not return multiple requests for comment.

Congressional Democrats have written a memo questioning Campbell's credibility and memory while Republicans say his story calls into question the favorable treatment the Obama administration gave Russia.

Notes of Campbell's FBI debriefings show he reported in 2010 that a Russian nuclear executive was using "the same kind of payment network" to move funds between Russia and Iran as was used to launder kickbacks between Moscow and Americans.

Campbell worked from 2008 to 2014 as an undercover informant inside Rosatom, Russia's state-controlled nuclear giant, while posing as a consultant. He helped the FBI put several Russian and U.S. executives in prison for a bribery, kickback, money laundering and extortion scheme.

He said he became concerned the United States was providing favorable decisions to the Russian nuclear industry in 2010 and 2011 - clearing the way for Moscow to buy large U.S. uranium assets and to secure billions in nuclear fuel contracts - even as he reported evidence of Moscow's help to Iran.

"I got no feedback. They took the reports and the reports, I assume, went to specific people assigned to analyze the reports and that was the last I heard of it," he said.

In 2012, FBI agents asked Campbell to press a top Russian nuclear executive about the Iran assistance, providing a list of detailed questions. The Russians became suspicious about Campbell's inquiries and fired him from his consulting job, he said

"It raised a red flag almost immediately and within a matter of weeks thank God I was out of harm's way," he said.