MEND spokesman Jomo Gbomo sent foreign news agencies a list with the names of the kidnapped workers - Italians Raffaele Pascariello, Alfonso Fravza, Ignazio Gugliota and Mario Celetano, US national John Stapleton and Croatian Jurica Ruic. They all work for the US company Chevron.
The Croatian foreign ministry previously stated that it knew the identity of the abducted Croatian and that his family had been informed about his kidnapping.
The MEND spokesman said that all hostages would be unconditionally released on May 30, but only if foreign companies did not try to secure their release by offering ransom.
Any such attempt will be considered bribery and will only compromise their situation, Gbomo said.
According to the spokesman, the purpose of the abduction was to warn the Anglo-Dutch oil giant Shell against returning to some oil fields in the Nigerian federal states of Bayelsa and Delta.
According to foreign news agencies, which have contradictory reports, the workers were abducted when MEND members attacked a tanker stationed offshore or an oil platform in the south of Nigeria. A Nigerian sailor was killed in the attack.