Last week three Nigerian policemen were killed during an attack on a Total facility at Obagi, in the southern Rivers state, in the most recent outbreak of violence against multinational oil companies.
 
The Niger Delta region, where all of Nigeria's crude oil is pumped, remains poor and underdeveloped despite the region's enormous oil deposits.
 
Many groups in the area say they are fighting to force the federal government to give their region a greater share in the oil wealth.
 
Oil output cut
 
The latest incidents bring the number of Filipino nationals being held hostage in southern Nigeria to 26. The other 24 were seized from a cargo vessel in Nigerian waters in January.
 
Their kidnapping prompted Gloria Arroyo, the president of the Philippines, to impose a temporary ban on Philippine nationals going to work in Nigeria, where nearly 4,000 are already employed.
 
More than 100 captives have been taken in the region since various groups stepped up their attacks on Nigeria's oil industry last year.
 
A year of violence in the Niger Delta has cut daily output by nearly a quarter and helped to raise the price of oil.
 
Despite the attacks on oil facilities, most hostages who are taken are released unharmed.
 
A seriously-ill British hostage was released on Wednesday and nine Chinese oil workers from the China National Petroleum Corp, kidnapped on January 25, were freed on Sunday.
 
Olusegun Obasanjo, the Nigerian president, has called a high-level meeting for Thursday to address violence in the region ahead of a general election in April.