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 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 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.