OSLO - Environmental campaigners have filed a lawsuit at an Oslo court challenging Norway's plan for seabed mineral exploration, citing insufficient impact assessment of the mining, the World Wide Fund for Nature (WWF) and its lawyer said on Friday.

Norway's parliament in January approved a proposal to open a vast ocean area larger than Britain for seabed mineral exploration after a government-commissioned study concluded that its impact would be minimal.

WWF, one of the world's largest conservation organisations, said it believed the study didn't meet minimum requirements set for impact assessment under Norway's Seabed Minerals Act, and therefore it provided no legal basis for the area opening.

"We believe that the state is violating Norwegian law when they now open up for a new and potentially destructive industry without having sufficiently investigated the consequences," Karoline Andaur, head of WWF Norway, said in a statement.

Deep-sea mining critics say industrial activity could destroy yet-to-be discovered species that live at the depths of some 1 500 m to  3 000 m and more, where seabed mineral deposits are located.

WWF also said Norway was sending a bad signal to other countries that could also decide to open their waters for deep-sea mining without proper impact assessment.

Norway's energy ministry has said the initial phase would allow a gathering of more information about the deep-sea environment and potential impact, and that companies would need separate permits to launch full-scale mining.

At least two companies expressed interest in specific ocean areas, or blocks, by a May 21 deadline, with the ministry planning to announce a first licensing round this autumn.

"We believe that a thorough process has been carried out with broad involvement, and that the applicable requirements have been followed," Astrid Bergmaal, state secretary at the energy ministry, said in an email to Reuters.

"I note that WWF wants to try the case in court, and they have the right to do so," she added.

A court hearing could begin within six months, a lawyer for the WWF said.