After Boko Haram in the northeast, a number of threats are destabilizing Nigeria's south.
The militants have threatened to blow up the ship and its crew if the pro-Biafran leader is not released.
A Nigerian court ordered the arrest of an ex-militant kingpin, prompting a spate of attacks on oil pipelines.
The ex-Niger Delta militant leader is accused of involvement in $175 million worth of corruption.
Buhari ahead with five states to go, but result could change with Niger Delta yet to declare.
Lawyers representing Nigerian farmers allege documents released by the UK courts show Shell lied in a previous case
Documents obtained by Amnesty show that the oil company ignored internal warnings that pipelines were in need of repair for six years before spills