The militants say they are fighting for greater local control of oil wealth
Militant groups in Nigeria's main oil-producing region, the Niger Delta, have stepped up their attacks in the area, claiming to have killed some 20 soldiers this week alone.
Nigeria is Africa's biggest oil producer but a wave of attacks earlier this year led to several oil facilities being shut and a 25% drop in its oil output.
What do the militants want?
They say that local people have not benefited from the region's oil, which has instead been used to develop other parts of Nigeria, or been stolen by the country's leaders.
It is true that Delta residents, like other Nigerians, live in poverty despite the riches under the ground.
The militants, under the umbrella Movement for the Emancipation of the Niger Delta (Mend), have also called for the release of militant leader Mujahid Dokubo Asari, who has been in prison for a year on treason charges.
Previous groups, such as Ogoni group Mosop, have also complained about the environmental destruction wrought by oil companies.
But some of the violence is criminal. They steal oil, which is sold on the black market, and kidnap oil workers to secure ransoms from the oil firms.
Who are the militants?
Mend is a shadowy group which communicates with journalists by e-mail.
They are mostly ethnic Ijaws - Nigeria's fourth largest ethnic group, who live primarily in the Delta.
Some of the militants first emerged after 2003 elections, claiming they had been recruited by local politicians to rig the polls and intimidate rivals but had not been paid.
Many believe that they retain links to some local and national politicians.
The black market in oil is extremely lucrative and some local military officials are also involved.
Two admirals were dismissed after an oil tanker with 10,000 tonnes of crude oil went missing.
Much of the oil is sold to eastern Europe, in exchange for weapons.
This explains how the militants are now able to take on, and even defeat, the military in a gun battle, rather than stage hit-and-run attacks, as they used to.
Why can't the government stop the violence?
The involvement of local politicians and military officials obviously makes things more difficult.
The terrain is also very difficult to operate in.
Extra soldiers have been sent to the Niger Delta
The Niger Delta is full of river creeks and swamps, which the militants know extremely well.
They enjoy some support from local people and can hide among them.
Previously, the army has been accused of being too heavy-handed, with civilians bearing the brunt, which has only further alienated the local population.
The government has held talks with some militant leaders, notably Mujahid Dokubo Asari, before his arrest but that too, failed to end the violence.
While it is never admitted, it is assumed that oil companies often pay ransoms for the release of their workers, which encourages more kidnappings.
The government and oil companies have tried to set up schemes to benefit local communities but the militants say this is not enough.
So what happens next?
Many fear an escalation of violence in the run-up to next year's elections.
Politicians could easily use the militants to stir up trouble again. This would make foreigners reluctant to go and observe elections there.
It could also suit opponents of President Olusegun Obasanjo to cause trouble to embarrass him.
Both the army and the militants have said they will send extra troops and fighters to the region.
In the meantime, the oil sector will continue to suffer.
Some oil workers have left the area, while security has been stepped up for those who are still there.