"Two ships are in Warri to protect oil installations and to prevent new violence in the area," navy spokesman Sinebi Hungiapuko told AFP.

An uprising by ethnic militants in the coastal swamps west of the city of Warri disrupted polling in last weekend's presidential elections, and last month forced oil multinationals to evacuate their facilities.

The US group ChevronTexaco, Anglo-Dutch giant Shell and France's TotalFinaElf had to suspend production because of the violence that erupted between security forces and militant ethnic Ijaws in the area.

At least 15 people, including oil workers and soldiers, were killed during the standoff before the situation was brought under control, and more than 3,000 refugees fled the swamps.

During the crisis Nigeria's exports were cut by more than 40 percent, and while they are now being progressively restored oil markets already troubled by the war in Iraq were made more nervous by the events.

Nigeria is the world's sixth largest oil exporter, with an OPEC quota of 2.018 million barrels per day.

Hungiapuko said that the two ships were the first in a batch of seven warships promised by the United States under its programme of military aid to Africa's biggest oil producer.

Earlier this month a US defence official told AFP the ships, rebaptised the Kyanwa and the Ologbo by the Nigerian Navy, were World War II-era patrol boats which have since been used by the US coastguard.

The 180-foot (60-metre) vessels have 42-strong Nigerian crews who were trained in the United States and were refitted at a cost to the US taxapyer of 750,000 dollars apiece before being handed over.

SPACE.WIRE