Seven Russian seamen among 12 captured during an attack on the German-flagged cargo ship BBC Palonia near the oil-rich Niger River delta, released, Voice of Russia said on Sunday.
The BBC Palonia vessel, flying the German flag, was attacked at about 21:00 Moscow time (17:00 GMT) on Friday near the southeastern Nigerian city of Bonny.
Along with Russians, two Germans were also released, as well as nationals of Lithuania, Latvia and Ukraine.
Earlier the editor-in-chief of the Maritime Bulletin magazine Mikhail Voitenko told the the Ekho Moskvy radio station that attacks on ships were a "normal practice" for many militant groups operating in Nigeria.
He said that though Nigerians generally treat their hostages "quite leniently," though they are believed to be crueler to their prisoners than Somali pirates.
Attacks on foreigners by militants, who demand that locals get more of the country's oil wealth, are frequent in the area since 2006. Kidnappings for ransom are also common.
Voice of Russia said there was no information of any ransom demands.
Earlier on Sunday the Russian Professional Union of Sailors said, referring to the ship-owners, that two Russian sailors from the North Spirit ship who were captured off Cameroon in mid-May and transferred to Nigeria have been released for a ransom.
The North Spirit vessel with a Russian-Ukrainian crew, flying the flag of St. Vincent and the Grenadines and owned by Greece's Balthellas Chartering S.A., was attacked on May 16 while anchored in Cameroon's largest port of Douala.
MOSCOW, July 4 (RIA Novosti)