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 sanctions came as Iranian state radio quoted the country's defense minister as saying that Iran's military capabilities had "increased remarkably" after the construction of a new submarine. The US 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. Earlier Tuesday, Iranian Defense Minister General Mostafa Mohammad Najjar was quoted as saying the new submarine was "smart," unmanned and had radar-evading capabilities. Tuesday's report did not elaborate on its size and technical specifics, and did not say whether the submarine had been tested. Iran occasionally announces production of advanced weapons. Since 1992, it has been active in producing military tanks, missiles, torpedoes, as well as guided bombs and airplanes. But little is known about Iranian submarines, beyond announcements years ago that Teheran bought some Russian subs and was to produce its own, smaller-sized ones.