Sixteen oil workers kidnapped in southern Nigeria's oil-rich Delta region have been released.
A range of gangs and militant groups operate in southern Nigeria
The workers were the second group of a total of 25 men to be released after being seized in a raid two days ago.
All worked for a sub-contractor of Anglo-Dutch oil giant, Shell. A spokesman said no ransom had been paid for their release.
Oil workers are regularly abducted in Nigeria by gangs saying they want a bigger share of oil revenues.
In a separate incident on Tuesday evening, five foreign oil workers were seized during a rare raid on a residential compound operated by ExxonMobil.
Two Nigerian security guards were killed when the gang stormed the compound in Akwa Ibom state in the eastern Delta.
A spokesman for Shell said that no money had been paid to secure the release of the 25 hostages seized on Monday.
Industry sources say oil companies frequently strike clandestine ransom deals to free their employees.
The men were taken when a group of about 70 militants attacked a convoy of boats supplying oil fields operated by Shell in Rivers state.
At least ten soldiers were killed during the raid.
The BBC's Alex Last says the line between criminal gangs and political militants in Nigeria is often blurred.
Last month, President Olusegun Obasanjo promised strong action to curb armed groups in the Niger Delta.
But our correspondent says the incidents highlights the continued vulnerability of the oil industry, despite government pledges to crackdown on armed groups in the Delta.
He says oil companies are bracing themselves for further violence in the Delta as competing politicians try to use the armed groups to secure victory in next year's elections.