The judge in the trial of Nigerian militant leader Henry Okah for gun-running and treason withdrew from the case on Friday because of accusations of bias, a move Okah's lawyers said represented a significant step forward.

"The judge has agreed to withdraw from the matter and he has forwarded the files for re-assignment," said Wilson Ajuwa, one of Okah's lawyers.

Nigeria's Director of Public Prosecutions, Salihu Aliyu, confirmed to Reuters that the Stephen Adah had withdrawn from the case after accusations of bias from Okah's legal team.

The move raises the possibility that the trial will be moved from the central city of Jos, where it has been held behind closed doors, to Bayelsa in the Niger Delta where the crimes were allegedly committed -- a key demand of Okah's legal team.

Okah is the suspected leader of the Movement for the Emancipation of the Niger Delta (MEND), the region's main militant group whose attacks on oil facilities have cut Nigerian output by more than a fifth over the past three years.

His lawyers say he is sick and in need of medical treatment overseas. MEND has been holding two British oil workers hostage in the Niger Delta for five months and has said their release depends on what happens to Okah.