Lagos (AFP) - Militants have blown an offshore oil facility operated by US oil group Chevron in southern Nigeria, the navy said on Friday, in renewed violence that could hit exports in Africa's largest oil producer.

"There was an attack on a Chevron facility near Escravos on Wednesday night. The incident happened about four nautical miles from Escravos, near Warri, in Delta state," spokesman Chris Ezekobe told AFP.

Ezekobe said "militants using explosives blew up the Okan platform, a collection facility for offshore oil and gas that feeds the Escravos terminal".

The navy was working with other security agents to track down the culprits, he added.

"A previously unknown group called the Niger Delta Avengers has claimed responsibility for the incident," the spokesman said, confirming a statement on the group's website.

"But we are not ruling out the involvement of former Niger delta militant leaders, particularly Tompolo, who is wanted on fraud charges."

A Chevron official, who asked not to be identified, confirmed the attack and said the facility had been shut down to contain spills.

There was no immediate indication of the volume of crude affected but the official said the attack would affect gas supply to power plants already hit with almost daily outages.

Attacks on oil and gas facilities have increased since January when Tompolo -- whose real name is Government Ekpemupolo -- was declared wanted on multi-million-dollar corruption charges.

The former leader of militants who wreaked havoc in the creeks and rivers of the delta in the 2000s is accused of defrauding the government of more than $175 million (161 million euros).

The offences linked to government maritime security contracts are alleged to have taken place between 2012 and last year, a court in Lagos has been told.

The upsurge in attacks is another security headache for President Muhammadu Buhari, who is battling Boko Haram Islamists in the northeast and an increase in violence between nomadic herdsman and farmers in central and southeast Nigeria.

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Tompolo, an ally of former president Goodluck Jonathan, was a prominent leader of the Movement for the Emancipation of the Niger Delta (MEND), which demanded a fairer share of oil revenue for local people, most of whom still live in poverty.

The Niger Delta Avengers group is thought to involve Tompolo's supporters unhappy about the charges against him and the winding down of a government amnesty programme that ended the unrest in 2009.

But Tompolo has previously said he is not part of the group.