Two Belgian firms are under investigation over allegations of illegally exporting nuclear material to Iran that can be used to make weapons, Belgium's energy ministry said Wednesday.

The companies sold zirconium powder and depleted uranium to the Islamic republic, so-called dual-use materials that can be used for military or civilian ends, said ministry spokeswoman Marie-Isabelle Gomez.

Energy Minister Paul Magnette said on RTL-TV1 that "before exporting nuclear material, even non-military material, a company must request a permit and authorisation. These two firms did not and we asked the proscutor's office to pursue the matter."

The energy ministry filed a complaint in 2008 against the two companies, which officials refused to identify, Gomez told AFP.

One of the firms failed to ask the energy ministry's permission to export zirconium powder, a mineral used in the reactors of nuclear power plants but which can also be used in bomb-making, she said.

The ministry consults a special panel on nuclear non-proliferation before granting or refusing the authorisation of such exports.

The second company did not inform the panel about its plan to export depleted uranium, which can be used to make armour-piercing artillery fire, Gomez said.

One of the firms, MDS Nordion, the Belgian subsidiary of Canadian group MDS Nordion, a producer of medical isotopes, said in a statement it had "always respected rules" and believed it had acted "responsibly".

Belgium till the year's end holds the rotating presidency of the European Union, which recently imposed fresh sanctions on Iran over its refusal to freeze uranium enrichment activities.

The United States and Europe suspect Iran is seeking to build an atomic bomb under the guise of a civilian nuclear programme, a charge Tehran denies.

Belgian Green parties want the ministries of energy, justice and foreign affairs to testify before parliament over the two firms' exports to Iran.

"It is important to be able to answer outstanding questions," said Green lawmaker Olivier Deleuze. "It is a question of credibility for Belgium."