ABUJA, Feb 6 (Reuters) - Nigerian gunmen abducted a Filipino worker and shot dead the policeman who was escorting him on the road to Port Harcourt in the oil-producing Niger Delta on Tuesday, a security expert working for a major oil firm said. The Philippines last month ordered the labour department to block workers from travelling to Nigeria after 24 Filipino seamen were captured on Jan. 20 in the Niger Delta, where kidnappings for ransom are an almost daily event. The 24 seamen are still in captivity.