US President Barack Obama has extended sanctions against Iran for one year, saying it continues to pose a threat to US national security.
In a message to the US Congress, Mr Obama said Iran was acting contrary to US interests in the region.
The sanctions, which ban US companies from investing or trading with Iran, have been renewed annually since 1995.
The US fears Iran's nuclear programme is a cover to build atomic weapons, a charge Iranian officials deny.
Enriched uranium can be used in nuclear power plants, but can also be used to make atomic weapons.
Although the Obama administration is reviewing its policy towards Iran, not extending the sanctions would have constituted a major break with the past, says BBC state department correspondent Kim Ghattas in Washington.
Iran faces other punishing international measures, including three sets of UN sanctions, over its refusal to freeze its uranium enrichment activities.
The US sanctions against Iran would have lapsed without Mr Obama's formal notice of renewal.
"The actions and policies of the government of Iran are contrary to the interests of the United States in the region and pose a continuing and unusual and extraordinary threat," Mr Obama's message to Congress said.
Mr Obama has talked of engagement with Iran but has not made clear how that might take place.
Shortly after coming to office in January, Mr Obama said "if countries like Iran are willing to unclench their fists, they will find an extended hand from us".