Factbox: Niger Delta's unending conflict

MEND said its previous attempts to cripple Nigeria's oil industry did not work and has appealed to another armed group, Niger Delta Avengers, to give peace a chance. The group urged the Avengers, who last week rejected a government truce offer, to join negotiations with authorities and to put down their weapons.

Ogoniland, a region of 404 square miles (1,050 km2) lies in the southeast of the Niger Delta basin. Economically viable oil deposits were first discovered there in 1957. It was just one year after the discovery of Nigeria's first commercial petroleum deposit in Bayelsa State, also in the Niger Delta.

The Ogoni people are a minority ethnic group of about half a million who call Ogoniland their home. The Ogonis and other groups in the region say that the government began forcing them to handover their land to oil companies without consultation or proper compensation shortly after the deposits were discovered.

In the 1970s and 1980s, the Ogoni's dissatisfaction grew. Environmental, social and economic conditions worsened, after assistance promised by the government failed to materialize.

Tensions between foreign oil corporations and a number of the Niger Delta's minority ethnic groups who felt they were being exploited, in particularly the Ogoni and the Ijaw, reached a high point in the early 1990s.

Ethnic and political unrest continued throughout the decade despite Nigeria's conversion from military rule to democracy and the election of the Obasanjo civilian government in 1999.

The competition for oil wealth fueled violence between many ethnic groups, causing the militarization of nearly the entire region by ethnic militia groups as well as by Nigerian military and police forces. In 2003 and 2004, local and state officials offered financial support to those paramilitary groups they believed would assist with the promotion of their own political agenda.

Meanwile, the oil industry was being targeted acts of piracy and kidnappings. However in 2009, the then Nigerian President Umar Musa Yar'Adua granted the oil militants an amnesty program. Those who renounced violence were offered financial assistance and job training. The program has been described by many as a success. Recently President Muhammadu Buhari announced he was going to wind down the program by 2018. This may have contributed to the re-emergence of oil militancy.

Calls for truce in Delta

The Niger Delta is home to five militant groups, MEND and Niger Delta Avengers are among them. There are still very few independently confirmed details about the Avengers, which announced its formation three months ago.

On its website, recently shut down, the group said it is fighting for an independent state on behalf of the people of the Niger Delta and is prepared to "cripple Nigeria's economy" in pursuit of its aims. Its tactic of attacking oil facilities in the region, early this year, has caused havoc in the sector, with production levels in the country now reported to have fallen to their lowest for more than two decades.

One attack on an underwater Shell pipeline in February showed a high level of technical expertise, forcing the shutdown of a terminal which normally produces 250,000 barrels of oil a day. Many locals suspect that some former oil militants excluded from the amnesty program could be behind the Niger Delta Avengers.