Congress is considering a new series of hard-hitting Iran sanctions on everything from mining and construction to the Islamic republic's already besieged oil industry, despite concern from the Obama administration that the measures could interfere with nuclear negotiations.
House and Senate bills are both advancing at a time President Barack Obama's national security team is gauging whether Iranian President-elect Hasan Rouhani is serious about halting some elements of Tehran's uranium enrichment activity.
Those involved in the process said the administration wants to temper Congressional plans until Rouhani takes office in August and has an opportunity to demonstrate whether his government will offer concessions.
The legislation would blacklist Iran's mining and construction sectors, effective next year, because they are seen as heavily linked to Iran's hard-line Revolutionary Guard corps.
It also would commit the US to the goal of ending all Iranian oil sales worldwide by 2015, targeting the regime's biggest revenue generator and prime source of money for its weapons and nuclear programs.
US penalties that went into effect last year already have cut Iran's petroleum exports in half, but that still leaves billions of dollars coming in every month from Turkey, China and several other Asian countries.
The House's bill may pass before Congress' August recess.
The Senate version won't get a vote until at least September, said Sen. Bob Menendez, chairman of the Senate Foreign Relations Committee and a leading advocate of tougher Iran sanctions.
The Senate Banking Committee, which will put forward the
package, is in ongoing consultations with the administration, according to one US official who, like others, spoke on the condition of anonymity because they were not authorised to talk publicly about the sanctions.
Republican Sen. John McCain said the US should immediately "plow ahead" with greater and tougher measures against Iran.
"We're running out of time," he said.
The US and many other countries believe Iran is seeking to develop nuclear weapons.
Obama has said Iran has until sometime next year to prove to the world its nuclear program is peaceful.
If diplomacy fails, the stage may be set for a military intervention by the US or Israel, which sees an Iranian nuclear weapon as a threat to its very existence.
In a report last week, David Albright and Christina Walrond at the Washington-based Institute for Science and International Security concluded that Iran will be able to produce enough weapons-grade uranium for a nuclear explosive without being detected by mid-2014.