WASHINGTON — The Bush administration has imposed economic sanctions on five Iranian companies that it has accused of helping the country pursue its ambitions to develop a nuclear weapon.

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

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 Company, and Joza Industrial Company.

“These five nuclear and missile entities have been used by Iran to hide its illicit conduct and further its dangerous nuclear ambitions,” the Treasury’s under secretary for terrorism and financial intelligence, Stuart Levey, 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 program.

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

Tehran insists its nuclear program is aimed only at generating electricity.

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