The Nigerian government says the navy of neighboring Benin is guarding a Greek-owned oil tanker hijacked by Nigerian militants demanding the release of a detained leader agitating for a breakaway state of Biafra.

PR Nigeria, which issues statements from the government, said the MT Leon Dias is anchored off Cotonou, Benin's commercial capital. It was unclear if the ship still has the crew and hijackers on board.

The vessel was hijacked Friday by separatists threatening to blow it up with its crew unless Nigeria releases Nnamdi Kanu, the director of the banned Radio Biafra who is accused of terrorism. Kanu's detention since Oct. 17 has sparked violent protests. The Biafran cause sparked a civil war that killed a million people in the 1960s.