Armed men have seized an oil tanker off Ivory Coast's main city, Abidjan, officials say.The Panamanian-flagged vessel, ITRI, was attacked as it prepared to deposit 5,000 tons of oil, the AP news agency reports.There has been a sharp increase in pirate attacks in recent years in West Africa.But most of these have taken place off the Nigerian coast, where oil is often stolen to be sold on the black market.