The ministry said the Hercules vessel, which was carrying 12 Russian and Ukrainian crewmembers, was seized on July 24. The gunmen later released the vessel along with seven crewmembers, and retained five men.
"The demands of the hostage takers and the location at which they are being held are not known," the ministry statement said.
However, the ship's owner, Sweden's Marine Carrier AB, has said the militants are holding four Russians and one Pole, according to Russian shipping news website Maritime Sovfracht Bulletin.
The website said the vessel seized at Port Harcourt in the in the Niger Delta was working for Italian oil services company Saipem SpA, a subsidiary of oil and gas multinational Eni SpA.
The militants, who belong to the Movement for the Emancipation of the Niger Delta, which has been linked to several attacks on foreign oil workers, are demanding that Saipem pay a ransom for the crew's release, the site said.
The Russian Embassy in Nigeria's capital Lagos has tightened security for staffers following the incident, the embassy press secretary said.
"In view of this situation, embassy staff are forbidden from independently travelling to the coast," Igor Popov said.
Russian embassy advisor Viktor Goncharov told the radio station Vesti-FM that the embassy is working closely with Nigerian law enforcers, who have established contact with the hostage-takers.
