The nine lawmakers, led by Republican Mark Kirk and Democrat Ron Klein, announced the move after The New York Times reported late Saturday that Washington has awarded more than 107 billion dollars in payments to foreign and US companies doing business in Iran despite US sanctions.

That sum included nearly 15 billion dollars paid to companies that defied US sanctions law by making large investments that helped Iran develop its vast oil and gas reserves, said the paper.

“The US government should be enforcing the Iran Sanctions Act, not rewarding firms that violate it,” said Kirk.

Kirk and Klein’s bill would toughen the 1996 Iran Sanctions Act (ISA), requiring President Barack Obama’s administration to investigate potential violators of the act and notify Congress of known offenders.

The 1996 law authorizes sanctions against non-US companies that invest more than 20 million dollars in Iran's oil and gas sectors.

In a letter to fellow lawmakers last week, Kirk and Klein noted that “while the original ISA was intended to deter investment in Iran's energy sector, no entity has ever been held accountable under the Act.”

The Iran Sanctions Enhancement Act would require the Government Accountability Office – the investigative arm of Congress – to publish a list of potential violators every month.

It calls for the president to conduct an immediate investigation based on that information and report back to the US Congress.

US lawmakers have stepped up calls on Obama to impose sanctions on Iran, as well as companies that do business with Tehran, in response to the Islamic Republic’s refusal to freeze its suspect nuclear drive.

Tehran denies Western charges that its atomic program hides an effort to develop nuclear weapons and has rebuffed UN demands that it halt uranium enrichment, which can be a key step towards building an atomic arsenal.

“Our legislation will put an end to mixed signals and bring the economic pressure necessary to allow diplomacy to succeed,” said Kirk.