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.The nationalities of those onboard are thought to be Filipino and Indian, with an Italian ship master.Commodore Kabir Aliyu, a spokesman for Nigeria's navy, declined to comment.Oil products tankers carry both crude and refined oil.The pirates are thought to have sailed into the Gulf of Guinea, according to security officials.The Halifax is owned by the Greek-based Ancora Investment Trust Inc and is flagged out of Malta.West Africa has seen a growing number of oil tanker hijackings in recent months as pirates target oil shipments from Nigeria, one of the world's biggest producers.On Monday, a top UN official, Taye-Brook Zerihoun, said that Somali pirates were inspiring copycat attacks in other parts of Africa.The warning came as the UN Security Council called on West African nations to step up navy patrols and other measures to counter a new front in the piracy war.