Gunmen in Nigeria have stormed a Dutch oil barge off the coast, killing two Nigerian navy sailors and kidnapping four foreigners, a naval officer says. The navy has joined the search for the gunmen off the Niger Delta. Two navy sailors, among a group providing security, were wounded in the raid. The foreigners are from Indonesia, Thailand, Malaysia and Iran. Attacks in the Niger Delta have declined since a 2009 amnesty for militants ended years of conflict. The ship that was attacked belongs to Sea Trucks Group, based in Rotterdam. The raid happened early in the morning, more than 30 nautical miles (56km) off the coast, the officer said. The heavily polluted delta is Nigeria's main oil-producing region. Oil output has risen since the amnesty took effect, but critics say the peace deal remains precarious. The militants targeted foreign oil companies, drawing on local support in a region still blighted by poverty despite huge profits for the multinational energy corporations. Local people wanted to see more of that wealth used to improve their housing and services. The BBC's Will Ross in Lagos says that despite the amnesty, kidnappings of Nigerians in the Niger Delta have been happening at an alarming rate. Piracy is increasing in parts of West Africa, he reports. The International Maritime Bureau says it has recorded 17 pirate attacks in Nigerian waters this year - a significant increase on 2011.