Nigerian separatists have hijacked a merchant ship and threatened to blow it up with its foreign crew if authorities do not release a detained leader agitating for a breakaway state of Biafra, military officers said Tuesday.

Maj. Gen. Rabe Abubakar, the Defense Ministry spokesman, confirmed the hijacking occurred on Friday and called it "an act of sabotage." He did not tell reporters the name of the ship.

Abubakar spoke on Monday. Other officers on Tuesday told The Associated Press that the navy is in pursuit of the captured vessel. The officers, who spoke on condition of anonymity because the issue is sensitive, said the hijackers have given the government 31 days to free Kanu or say they will blow up the ship along with its crew.

The ultimatum was given at the weekend by a militant identified by the nom de guerre of General Ben. Ben is not a separatist but "some Niger Delta militants have shown interest in working with us," said Uchena Madu, a leader of the Movement for the Actualization of a Sovereign State of Biafra.

The hijacking indicates the separatists could be working with some Niger Delta oil militants blamed for recent bombings of oil pipelines in the oil-rich south, escalating conflict in a country already burdened by Boko Haram's deadly Islamic uprising in the northeast and violent ethno-religious confrontations between farmers and herders in central Nigeria. Africa's biggest economy and oil producer also is battered by slashed petroleum prices.

Secret police on Oct. 17 detained Nnamdi Kanu, director of banned Radio Biafra, and since have accused him of terrorism, sparking protests in which police are accused of killing several demonstrators.

Nigeria's Igbo people prosecuted a civil war to create a separate state of Biafra in the southeast that killed a million people in the 1960s. Many Igbos charge they still suffer discrimination.