The US is preparing to designate Iran's Revolutionary Guards force as a foreign terrorist unit, officials say.
If confirmed, this will be the first time official armed units of a sovereign state are included in the list of banned terrorist groups.
The classification would allow the US to target the force's finances.
The US has repeatedly accused Iran of destabilising Iraq and Afghanistan, blaming the Revolutionary Guards for supplying and training insurgents.
There are currently 42 organisations on the state department's list of foreign terrorist organisations.
They include al-Qaeda, the Lebanese militant group Hezbollah and the Palestinian groups Hamas and Islamic Jihad.
'Price to pay'
Administration officials told the BBC and US media that the decision to list the Revolutionary Guards was close to being announced.
The move was intended to increase the pressure on Iran amid growing concern in Washington that Tehran is funding and supporting insurgents in Iraq and the Taleban in Afghanistan, as well as groups such as Hezbollah in Lebanon, the officials said.
The BBC's Jonathan Beale, in Washington, says that by listing the guards as a terrorist group the US would be sending a warning to governments around the world not to have any dealing with the Revolutionary Guards or their business interests.
Secretary of State Condoleezza Rice was behind the move to add the Guards to the list, the New York Times reported.
Ms Rice has championed a diplomatic approach to relations with Iran over the past 12 months.
But she was said to have backed the plan after progress on tougher sanctions against Iran over its nuclear programme by the UN Security Council became bogged down, the Times said.
Speaking at a news conference last week, President George W Bush hinted at a tougher stance against the Revolutionary Guards, accusing them of meddling across the Middle East.
"When we catch you playing a non-constructive role, there will be a price to pay," Mr Bush said.
The BBC News website's world affairs correspondent, Paul Reynolds, says the unanswered question is whether the new American move would be another step on a path to a military strike against Iran's nuclear facilities.
The Revolutionary Guards force was established after the Islamic revolution toppled the Shah and brought hard-line clerics to power in Iran in 1979.
It is estimated to have 125,000 active members, and operates separately from Iran's main armed forces, boasting its own ground forces, navy, air force, intelligence and special forces capability.
Кратенько - США готовятся к внесению КСИР в список террористических организаций, от чего даже верные союзники- англичане пребывают в некотором недоумении. В ближайшее время ожидается рассылка уведомления о внесении КСИР в список террористов правительствам всего мира.
