A Nigerian court on Saturday annulled the governor's election in the country's oil hub Rivers State due to irregularities and ordered a fresh vote within three months in a ruling likely to add to tensions in the sensitive region.
Former militants in the southern region have said they might resume a fight for a greater share of oil revenues if President Muhammadu Buhari ends an amnesty due to expire in December.
A special election petition tribunal in the federal capital Abuja declared void the April election of Nyesom Wike of the Peoples Democratic Party (PDP) as state governor.
"The process of accreditation for the election was compromised," tribunal chairman Justice Suleiman Ambursa said. "It is the view of the tribunal that the election was held contrary to guidelines."
The PDP of former President Goodluck Jonathan, who lost the presidential polls to Buhari in March, rejected the ruling as "manipulation" by Buhari's All Progressives Congress (APC).
The ruling was "completely bizarre, unacceptable and part of the script by the APC to manipulate the will of the people," the PDP said in a statement.
Wike's lawyer Chris Uche said he would appeal the ruling. Buhari's APC had filed a complaint against the state governor vote.
Tensions have been building up in the Niger Delta, to which Rivers State belongs, which remains impoverished despite sitting on much of the West African nation's oil wealth.
Buhari has said he wants to overhaul a multi-million amnesty programme for former militants giving them cash handouts and training to get jobs and stop them attacking oil pipelines, but has left details open.
Jonathan, who is also from the regions, had the support of most in the Christian Niger Delta during the March polls. Buhari hails from the predominantly Muslim north.
Africa's biggest economy is in a policy vacuum as the Senate has not yet approved Buhari's cabinet, which the former military ruler took four months to nominate.