Bletyn Wikina, a Rivers state official, said the German man was with Nigerian authorities and was unharmed.

The man, whose name was not released, was snatched on August 3 from a makeshift checkpoint by gunmen wearing military fatigues.

Wikina did not say if a ransom had been paid and it was unclear if the release was linked to a new campaign against those targeting foreign oil workers and the oil industry in the area.

The campaign was announced by Olusegun Obasanjo, the Nigerian president, on Tuesday and military officials said it began on Friday.

Gunfire

Soldiers searched houses in the main Niger Delta city of Port Harcourt on Friday and detained at least five people.

The soldiers fired into the air for at least an hour, causing panic in the streets.

Sagir Musa, a military spokesman, said: "This is the beginning of something bigger to drive all the bandits from the state. It will continue. It is ongoing."

Discontent

A series of kidnappings have rocked Port Harcourt with 15 foreigners seized in the streets or taken from nightclubs over the past two weeks.

Ten have been released unharmed but five remain unaccounted for.

Attacks on the oil industry have cut Nigeria's daily oil production by nearly a quarter from its normal 2.6million barrels. The country is Africa's biggest oil exporter.

Many Nigerians say the violence in the region stems from popular discontent over the portion of Nigeria's oil earnings that returns to their areas.