It was the third major attack on an oil facility in Africa's largest producer since the arrest of a militant leader in September, but exports from ExxonMobil's 400,000 barrel-per-day Qua Iboe terminal were not immediately affected.

"There is an ongoing attack at Qua Iboe terminal by militants in up to seven boats. The navy is reported to be repelling the attackers," one of the sources said, asking not to be named because he is not allowed to talk to the media.

A spokeswoman for ExxonMobil said there were reports of shooting around Eket, the town closest to Qua Iboe, and workers at the terminal were in lockdown but operations were continuing.

"We have unconfirmed reports of shooting all around the Eket area. Our response was to go into lockdown and for now operations are ongoing," Gloria Essien said.

Militants whose attacks have cut Nigerian oil output by one-fifth ended a five-month ceasefire in September when militia leader Henry Okah was arrested on arms trafficking offences in Angola.

They had called off attacks when President Umaru Yar'Adua was inaugurated in May to give him a chance to make good on a promise to address grievances of poverty and neglect in the vast wetlands of southern Nigeria.

But Okah's group, the Movement for the Emancipation of the Niger Delta (MEND), announced the end of the ceasefire on September 23.

Gunmen kidnapped 13 oil workers from two different offshore oilfields in October. All were released after a few days. MEND claimed reponsibility for both abductions.

The government's attempts to lure militants to the negotiating table has split them into factions, and activists in the region say the resurgence of attacks is also a symptom of a struggle for supremacy.

MEND's Okah has been in a war of words with rival militia leader, Mujahid Dokubo-Asari, who has agreed to join the government's peace talks and spoken out against kidnapping.