Viktor Gorcharov, a counselor at the Russian embassy in Nigeria, said the two Russians, two Ukrainians and a Pole were released in a special operation by Nigerian law enforcement agencies in the early hours of Monday morning.
"They are fine; they have no injuries, wounds or signs of trauma," he said.
Maritime Sovfracht Bulletin said the Hercules tugboat, owned by Sweden's Marine Carrier AB, was working for Italian oil services company Saipem SpA, a subsidiary of oil and gas multinational ENI SpA.
The vessel, which had 12 crewmembers, was seized last Thursday. The gunmen later released the vessel along with seven crewmembers, while keeping hold of five men.
The militants from the Movement for the Emancipation of the Niger Delta, which has been linked to several attacks on foreign oil workers, demanded that Saipem pay a ransom for the crew's release.
The Russian Embassy has tightened security for staffers following the incident, the press secretary said.
