Nigerian militants have blown up an oil pipeline near US corporation Chevron’s Escravos oilfields, the Nigerian military says.
Chevron said the attack prompted it to shut down onshore oil production.
The loss equates to about 120,000 barrels per day, about 6.6% of Nigeria’s total daily crude production.
Earlier this week Nigeria’s president ordered tighter security in the Niger Delta, after an attack on Shell’s main offshore facility.
According to the BBC’s Alex Last, in Lagos, sources in the western Niger Delta believe the latest attack is the work of illegal oil “bunkerers” – involved in the lucrative trade in stolen oil.
The attack on Shell’s floating oil platform at Bonga, which cut a tenth of Nigerian oil production in one go, was carried out by militant group the Movement for the Emancipation of the Niger Delta.
But the Escravos incident also highlights the vulnerability of the oil infrastructure in Nigeria, our correspondent says.
With the government planning to hold a big summit of Niger Delta leaders and more money expected to flow to the Niger Delta, perhaps the armed groups there feel it is a good time to show how relevant they are to any chance of peace, our correspondent adds.
While the loss to Nigerian crude output is significant, it is a small fraction of the daily global oil output, of about 85 million barrels per day.
News of the Escravos attack, carried out on Thursday, comes ahead of a meeting in Jeddah, Saudi Arabia of major energy producers and users – where the rising price of oil will be the key topic for discussion.
On Friday the Nigerian government announced how it would spend a $10bn (�5bn) windfall from the rising oil price.
It will spend just over $5bn fixing the country’s power supply and the rest will be shared among the 36 state governments.