Militants on Monday abducted about 25 Nigerian staff working for a contractor and killed at least 10 soldiers in a raid on a convoy of boats supplying oilfields in the Niger Delta.
   
No production was affected, the company said.
   
On Wednesday, the Shell spokeswoman said: "All the contractors have been released."

Attacks and pipeline leaks have cut roughly a quarter of production for Africa's biggest oil producer.

More kidnappings

In a separate attack on Tuesday, Nigerians suspected of campaigning for a greater share in the country's oil wealth, invaded a residential compound for foreign oil workers, killing two guards and kidnapping at least five expatriates.
   
The workers were taken from the compound in Eket, close to the operational base of Exxon Mobil, which exports about 800,000 barrels a day from Africa's top oil producer.
   
The series of attacks ended a period of relative quiet in the Niger Delta, which accounts for all oil output from the world's eighth biggest exporter.

Violence in the delta is a result of widespread poverty, corruption and lawlessness.

Most inhabitants of the wetlands region have seen few benefits from five decades of oil extraction that has damaged their environment.