Gunmen attacked a houseboat in Rivers State in the Niger Delta Wednesday killing a naval rating and a civilian crew member, the army said.

Spokesman for special troops deployed in the region to provide security for oil workers and facilities, Lieutenant-Colonel Sagir Musa, said about 30 militants attacked the boat, after which there was a shoot-out with navy personnel.

'Three militants were killed by navy personnel guarding the boat. A naval rating and a civilian staff member on the boat were also killed,' he said.

Musa said the incident took place at Olubiri, near Okrika in Rivers state. He praised the navy men who, though few in number, had fought off the heavily-armed militants.

Meanwhile, the German-founded Julius Berger Nigeria Limited construction company was reported by witnesses to have pulled out of Rivers and Bayelsa States, both in the Niger Delta, following the abduction of two German employees whose whereabouts remained unknown.

None of the militant groups has claimed responsibility for the abduction, but an employee said that the kidnappers had made a ransom demand of 25,000 dollars.

'All the expatriates that you are seeing around only come to see the situation and return to their hotels afterwards,' the employee said. 'Julius Berger is not prepared to pay and has pulled out actually. That is why there is no work going on here.'

The unidentified employee said the company would not work in any part of the Niger Delta until its employees were released.

Restiveness in the Niger Delta has led to the loss of a quarter of Nigeria's daily crude oil output of 2.1 million barrels, contributing to spiralling oil prices on internationa markets.