Gunmen in Nigeria`s oil-rich Niger Delta have hijacked a ship belonging to an US company and kidnapped its crew, according to BBC.

The militants are demanding a ransom of 30m naira ($259,000) for the boat and the crew, a military spokesman said.

The vessel, which ferries supplies for the oil company Chevron, was hijacked on Tuesday night near Port Harcourt.

Nine Nigerians, a Portuguese and a Ukrainian were sailing the vessel when the gunmen boarded it, according to military spokesman Lt Sagir Musa.

Nobody has so far claimed responsibility for the attack.

Nigeria is Africa`s biggest oil producing nation, but frequent pipeline bombings and other attacks by militants demanding a bigger share of the profits have reduced output by 25%.

The remote and inaccessible creeks of the Delta are also home to criminal gangs who seize oil workers and attack pipelines for money.

Piracy in Nigeria`s waters has also increased in the last two years, making its the most dangerous shipping area in the world.