The US Treasury Department on Monday said it would announce "new steps" to pressure Iran amid renewed suspicion about Tehran's nuclear ambitions.
Secretary of State Hillary Clinton and Treasury Secretary Tim Geithner are expected to make an announcement of further sanctions around 2130 GMT.
The two will "outline new steps the United States is taking to increase pressure on Iran," a Treasury announcement said.
The measures are expected to target Iran's financial, oil and petrochemical sectors, after a report by the UN atomic energy watchdog strongly suggesting Iran was trying to build nuclear weapons.
They also follow US allegations last month that Iranian officials were involved in a thwarted plot to kill the Saudi ambassador to Washington.
The move appears to be part of a coordinated effort by Western governments to isolate Iran in the absence of further multilateral UN sanctions.
Earlier on Monday finance minister George Osborne said Britain was cutting links with Iranian banks because of "evidence" that they are involved in the development of a nuclear weapons program.
"The UK government has just announced new financial sanctions against Iran. We are ceasing all contact between the UK financial system and the Iranian banking system," Osborne said.
"Today's announcement is a further step to preventing the Iranian regime from acquiring nuclear weapons."
Iran denies Western claims it is developing nuclear weapons.
It is not clear if the US measures will target Iran's Central Bank, a move some experts say could have a profound impact on Iran's economy and global energy prices.
US and European officials have said they worry that hitting the central bank with sanctions could send oil prices abruptly soaring as Iran's big oil exports would have a harder time getting to market.
There was little market reaction to the news.
Oil prices in New York and London were pushed lower by fears about the economy and debt levels in the United States and Europe.