Nigeria's state oil company said on Wednesday it had shut down two of its four refineries due to crude supply problems after recent pipeline attacks.
The Nigerian National Petroleum Corporation carried out an "operational shutdown" of the refineries in the northern city of Kaduna and Port Harcourt, in the southern Niger Delta, on Sunday, it said in a statement.
It was unclear how long the refineries would be shut. A spokesman declined to give details.
The shutdown follows a weekend of attacks on pipelines in the Niger Delta, the oil hub in Africa's top crude producer.
"The plants were shut simultaneously on Sunday after the Bonny - Okrika crude supply line to the Port Harcourt refinery and the Escravos-Warri crude supply line to the Kaduna refinery suffered breaches," Nigeria's state oil company said.
The attacks follow years of relative calm in the country's oil-producing region after a 2009 amnesty halted a spate of attacks on oil installations and kidnappings of expatriate workers.
Late on Tuesday, Nigeria's military issued a statement saying those behind the weekend attacks would "no longer be tolerated", adding that pipeline vandals would "face the full weight of the law".
Despite being Africa's largest crude exporter, Nigeria imports almost all of its gasoline. Officials had hoped newly revamped national refineries would produce up to 30 percent of its gasoline needs in the first quarter of this year.
Nigeria, an OPEC member that relies on oil exports for around 95 percent of its foreign earnings, is facing its worst economic crisis in years and dwindling foreign reserves as a result of the nosedive in global crude prices.
(Reporting by Alexis Akwagyiram)