The LPG vessel has been stationed on the Bonny river for two years
Eight foreign oil workers, kidnapped by Nigerian militants, have been freed unharmed, an army spokesman says.
In the early hours of Saturday, a group of gunmen in a speed boat attacked a petroleum tanker on the Bonny river in the south of the country.
Two people were shot and injured while eight oil workers, believed to include a number of Russians, were seized.
"They have been released," about 2030 (1930GMT), said Lt Col Sagir Musa. "I doubt any ransom was paid."
The nationalities of those seized have not yet been confirmed, and no group has claimed responsibility.
More than 200 foreign oil workers have been kidnapped in the Niger Delta over the past two years but often released after payment of ransom.
It is believed the tanker which came under attack belongs to Global Gas and Refining Ltd, a Nigerian subsidiary of US-based Global Energy Inc, which has been stationed along the Bonny river for more than two years.
Lt Col Musa, military spokesman in the eastern Niger Delta, said earlier: "Around six heavily armed bandits attacked an LPG (liquefied petroleum gas) tanker, shot two civilians and abducted eight of the expatriates, whose identity is not yet ascertained."
He said the two civilians had been wounded but not killed during the attack, which took place between 0100 and 0400 (0000-0300 GMT).
Late on Thursday, 12 people were kidnapped from a boat near the Niger Delta. Seven of them were later freed but five people remain captive.