Nigerian security agencies have stepped up efforts to trace the four foreigners who were kidnapped when an oil company vessel was attacked off the country's coast.
Suspected pirates stormed a vessel belonging to the Sea Trucks Group in the Gulf of Guinea, scene of a series of maritime attacks in recent months.
Two people were killed during the assault, which took place 35 nautical miles off Nigeria's oil-rich Niger Delta coastal region.
"We have intensified our search for the kidnappers and the abducted four foreigners," said Nigerian Navy spokesman Commodore Kabir Aliyu.
A second official, speaking on condition of anonymity, said an investigation of the Gulf's subsidiary creeks and waterways is also under way.
"We are leaving no stone unturned in our efforts to get back these four foreigners who were kidnapped aboard the vessel. We are redoubling our efforts," he said.
A spokeswoman for Sea Trucks Group said the company's main goal was the safe release of the hostages.
"We are very focused on getting our crew back safely," Corrie Van Kessel said.
Van Kessel confirmed the four people kidnapped were from Indonesia, Iran, Malaysia and Thailand.
The Gulf of Guinea has been plagued by armed insurgency from militants in recent years, following widespread criticism of companies' refusal to redistribute oil revenues among local communities.
From 2006 to 2009, armed gangs targeted a number of oil companies, leading to a fall of over 28 percent in the production of crude oil.
Despite a 2009 amnesty deal that reduced the violence, the region has seen a fresh rise in the number of reported pirate attacks in 2012.
In a report released in July, the International Maritime Bureau (IMB) reported that 32 incidents of piracy had been recorded off the coasts of Benin, Nigeria and Togo in the first half of 2012, up from 25 in 2011.
An IMB official told AFP that armed assaults on vessels in the area are being been under-reported.
Many of the raids include "high levels of violence," the IMB report added.