Talks under way
The ship was near the Nigerian oil city of Warri when the armed men took 17 sailors to a nearby village on Saturday and forced the remaining crew members to stay on board, Esteban Conejos, the Philippine foreign affairs undersecretary, said on Tuesday.
The Philippine government had initially said six sailors had been kidnapped.
Local Nigerian government officials had already identified a negotiator and talks were under way, Conejos said, adding that all the Filipinos were apparently safe and sound.
The armed men are from the Movement for the Emancipation of the Niger Delta, which has been seeking the release of the movement's leader and two other men charged with treason, he said.
Unaware of demands
Klaus Steffen, manager at Baco-Liner GmbH of Duisburg, Germany, said his company had no contact with the ship's crew and could not confirm that seven crew members remained on board.
Conejos said he was unaware of any specific demands from the group in the current negotiations.
The group has launched crippling assaults against the energy industry in Africa's oil giant since last year.
Two days before the seizure of the Filipino crew, the group released five Chinese and one Italian hostage, seized earlier in separate incidents in the oil-rich southern delta region.
The Nigerian government was fully co-operating with the Philippines to win the hostages' release, Conejos said.
"Both of our governments are dead set to resolve this incident peacefully," he said.
The ship, Baco-Liner 2, remained anchored off Warri under the captors' control, he said.
The ships cooks have remained on board and are making food that is being taken to the hostages in the village, he said.