Nigerian rebels have demanded $40 million for the safe return of four kidnapped British oil workers.

Several foreigners kidnapped on Tuesday night remain in captivity. The hostage takers, who haven't been identified, have demanded $10 million each for the captive foreigners, according to a Western diplomat.

Aberdeen-based Sparrows Offshore have confirmed three of their employees were among four Britons abducted in an attack on a residential compound in Akwa Ibom state.

Troops began a search-and-rescue mission today for Nigerian soldiers missing after militants ambushed a military-escorted supply convoy in the restive, oil-rich south, officials said.

Brig Gen Alfred Ilhogo confirmed the late-Wednesday attack on the convoy bringing provisions to Agip, a subsidiary of Italian oil company Eni SpA, in the Niger Delta region, where much of the crude in Africa's largest producer is pumped.

He had no casualty figures, but said some soldiers were missing after the attack and that rescue operations had been launched today.

A lower-ranking officer,said nine out of 11 soldiers escorting the convoy had failed to return to base.

A series of attacks and kidnappings in Africa's largest oil producer have cut Nigeria's production by more than a quarter so far this year.