It is the latest in a series of attacks on oil facilities in Africa’s top oil exporter. President Muhammadu Buhari has vowed to crack down on “vandals and saboteurs” in the Delta region, which produces most of the country’s oil.

In a statement, the energy company said Chevron Nigeria Limited, operator of a joint venture with the Nigerian National Petroleum Corporation, said an attack took place at about 11.15 pm (1715 ET) on Wednesday.

“Its Okan offshore facility in the Western Niger Delta region was breached by unknown persons,” said Chevron in the statement. “The facility is currently shut-in and we are assessing the situation, and have deployed resources to respond to a resulting spill.”

There were no immediate details of any casualties. The company could not be reached for further comment.

A group known as the Niger Delta Avengers claimed responsibility for the attack, and in a statement it said it blew up the platform.

“This is what we promised the Nigeria government since they refuse to listen to us,” the group said.

The same group has said it carried out an attack on a Shell (RDSa.L) oil pipeline in February which shut down the 250,000 barrel-a-day Forcados export terminal.

The militants say they want a greater share of oil revenues. Crude sales account for around 70 percent of national income in Africa’s biggest economy.

Pipeline attacks and violence have risen in Nigeria’s southern swampland since authorities issued an arrest warrant in January for a former militant leader on corruption charges.

Buhari has extended a multi-million dollar amnesty signed with militants in 2009 but upset them by ending generous pipeline protection contracts.

The militancy is a further challenge for a government faced with an insurgency by the Islamist militant Boko Haram group in the northeast and violent clashes between armed nomadic herdsmen and locals over land use in various parts of the country.