"The new sanctions include everything up to and including prohibiting these entities from doing business in the United States," Democratic Senator Evan Bayh, one of the bill's lead authors, told a press conference.
The legislation looks to tighten the screws on the Islamic republic, which imports about 40 percent of its gasoline, by targeting firms that finance gasoline exports to Iran, or either ship or insure the exports.
"Bottom line: It allows us to put our finger right on the pressure point where Iran is the weakest, and that is gasoline," Democratic Senator Chuck Schumer told reporters.
The measure would grant the US president powers to impose a range of sanctions on such companies.
"We are giving the president a powerful new weapon to use in the negotiations with Iran, but it is up to him to decide when, where and against whom to use it," said Independent Senator Joseph Lieberman.
Similar legislation was being written in the House of Representatives, where another bill introduced last week would require the imposition of sanctions.
Oil-rich Iran relies on the imported gasoline to help meet domestic demand, with most imports coming from five European firms and one Indian company.
Entities potentially affected include the Swiss firm Vitol, the Swiss/Dutch firm Trafigura, France's Total, the Swiss firm Glencore and British Petroleum, as well as the Indian firm Reliance, while Lloyds of London insures the majority of tankers carrying gasoline to Iran.
"You can either do business in our 13-trillion-dollar economy, with us, or you can do business with Iran, with its 250-billion-dollar economy, but you can't do both," said Republican Senator Jon Kyl.
The legislation came amid mounting frustration in the US Congress over the pace of progress in diplomatic negotiations, backed by global sanctions, aimed at convincing Tehran to freeze sensitive nuclear work that can lead to an atomic bomb.
"We are hoping to make it more likely that that diplomatic engagement with the Iranians will succeed and that they will peacefully abandon their nuclear ambitions because engagement cannot be an open-ended process," said Lieberman.
"Engagement is a tactic. It's not a strategy. This engagement has to be part of a broader strategy if it's going to work that has goals and schedules and teeth. And this legislation, clearly, provides those teeth."
