Pirates have seized an oil products tanker off southern Nigeria, security officials say.

Gunmen boarded the MT Halifax off Port Harcourt, the main city in the oil-rich Niger Delta, they said.

The owners of the vessel lost contact with the tanker on 30 October, according to the International Maritime Bureau (IMB).

The UN this week urged West African nations to increase naval patrols following a surge of pirate attacks.

The IMB says the crew of 25 are being held by the pirates, who are expected to steal the oil in the ship's hold.

In West Africa, crews are generally released unharmed after the crude oil is stolen from the ships, unlike in Somalia, where crews and their ships are held until ransoms are paid - often after several months.

http://www.bbc.co.uk/news/world-africa-15576373