A militant group active in oil-rich southern Nigeria says it has bombed three oil pipelines in Bayelsa State.
Mend threatened to step up their attacks after the elections
The Movement for the Emancipation of the Niger Delta (Mend) has carried out a number of attacks on oil facilities and abducted foreign workers.
The latest - eight Filipinos and three South Koreans kidnapped last week - were freed on Tuesday, sources said.
Mend wants a larger share of oil wealth. Its attacks cut production in Nigeria by more than 25% last year.
The price of oil in London is now up to more than $65 a barrel.
Government officials confirmed that the attacks had taken place in the Brass and Akassa areas without giving details of the damage caused.
Formed early 2006
Close links to militant Mujahid Dokubo-Asari's Niger Delta Volunteer Force
Split into two rival groups late 2006
Bayelsa State faction leader - Jomo Gbomo
Delta State faction leader - Gen Godswill Tamuno
Demand 100% control of Nigeria's oil wealth
Demand release from jail of Dokubo-Asari being tried for treason
Demand release of impeached Bayelsa governor on trial for money laundering
Operate from creeks of Niger Delta
Communicate with media by email
But a spokesman for the Italian oil company Eni told Reuters the attacks forced it to stop producing 150,000 barrels a day at its Brass export terminal.
The BBC's Alex Last in Nigeria says the pipelines are easy targets as an attack on a single pipeline can significantly affect production and exports.
The militants had earlier warned that they would step up attacks on oil installations in the region as "parting gift" to President Olusegun Obasanjo who stands down in three weeks time.
They say the attacks were also in pursuance of their resolve to cripple the Nigerian crude oil export industry.
The militants say they will continue their renewed campaign "indefinitely with attacks on all pipelines, platforms and support vessels".
The eight Filipinos and three South Koreans, all workers at Daewoo, had been taken at gunpoint from a heavily guarded compound last Thursday.
Their Nigerian driver was later released.
South Korean government officials said the freed hostages were well.
Nearly 100 foreign workers have been abducted this year.