The US Senate unanimously approved on Monday a package of new economic sanctions on Iran's oil sector just days ahead of a meeting in Baghdad between major world powers and Tehran.
The sanctions add to a raft of punitive measures by the United States and the European Union aimed at shrinking Iran's oil revenues to force it to halt a nuclear programme the West suspects is being used to build an atomic bomb.
Iran has said its nuclear programme is for civilian purposes.
The new package would extend sanctions to cover dealings with the National Iranian Oil Co and National Iranian Tanker Co, if they are deemed to be an agent or affiliate of the Revolutionary Guards. It aims to close a potential loophole that could have allowed Tehran to continue selling some of its oil using its own fleet.
The House of Representatives passed its version of the bill in December and now the Senate and House must work out their differences in the legislation before it is signed into law by the President.
"This bill is another tool that will demonstrate to Iran that the US is not backing down," Robert Menendez, the Democratic senator who helped craft the legislation, said.