Legislation introduced by a bipartisan group of U.S. senators would block Iran's access to billions of dollars worth of foreign currency reserves.
This was the latest congressional effort to slow development of the Islamic Republic's disputed nuclear program.
Lawmakers in Washington say the government in Tehran taps the reserves held in banks around the world, mostly in Euros, to get around U.S. and EU sanctions on oil sales that have damaged Iran's economy.
Iran converts the reserves it built up from decades of selling oil, estimated to be worth $60bn to $100 bn, into local currencies in order to finance imports and stabilize its budget, the lawmakers say.
The US, Israel, and the EU believe that Iran is enriching uranium to levels that could be used in nuclear weapons. Tehran has rejected the claims.
The US and EU, based on undemonstrated claims, have put sanctions on Iran. If passed, the bill would block such currency conversions of the reserves and be retroactive to May 9.
The bill is expected to be attached later this month to Iran sanctions legislation in the House of Representatives that was introduced in February by Ed Royce, the chairman of that chamber's foreign affairs committee.