WASHINGTON —  The Bush administration has imposed economic sanctions on five Iranian companies that it has accused of helping Iran pursue what the United States considers the Islamic Republic's ambitions to develop a nuclear weapon.

The Treasury Department announced Tuesday that it was freezing any assets the five companies might have in the United States and prohibiting American individuals and companies from having dealings with the five.

The five companies are the Nuclear Research Center for Agriculture and Medicine, the Esfahan Nuclear Fuel Research and Production Center, Jabber Ibn Hayan, the Safety Procurement Co. and Joza Industrial Co.

"These five nuclear and missile entities have been used by Iran to hide its illicit conduct and further its dangerous nuclear ambitions," Stuart Levey, Treasury's undersecretary for terrorism and financial intelligence, said in a statement announcing the action.

The new sanctions represent the latest effort by the administration and its allies to increase pressure on Iran to halt its nuclear enrichment program.

The European Union tightened trade restrictions Friday on Iran, and the West has threatened a fourth round of sanctions over Tehran's refusal to halt uranium enrichment. The process can produce fuel either for nuclear reactors or weapons.

Iran insists its nuclear program is meant only for generating electricity.

The actions taken by the administration on Tuesday were under an executive order President George W. Bush signed to target entities accused of aiding in the proliferation of weapons of mass destruction.