Industry sources said the men were employed by HydroDive (Nigeria) Ltd, an offshore engineering company subcontracted to Hyundai Heavy Industries Co Nigeria Ltd.
 
The kidnapping brings to nine the number of foreigners currently held in the Niger Delta, a region of swamps and creeks that is difficult to police.
 
On Saturday, an American oil worker, taken hostage last month, was released along with his local driver, according to police and company sources.
 
A source at the men's employer, Nigerian construction company Pivot, said: "They were released at 10 last night. They are in hospital."
 
Twenty-four Filipino sailors, taken hostage in January, were also recently released by the armed group that was holding them.
 
Since the start of this year alone 55 foreigners have been kidnapped by armed groups in southern Nigeria, almost as many as for the whole of 2006.
 
Most of those taken hostage are released unharmed.
 
Many in Nigeria's southern delta region, which produces all of the country's oil, live in poverty and some of the armed groups involved in the kidnappings say they are calling on the government to give the region a greater share of the country's oil wealth.
 
Hostage-takings for ransom have become increasingly common.