Southern Nigeria's main armed group MEND announced Sunday it had freed a Ukrainian and three Nigerians who were captured by pirates in mid-September.
The hostages were part of a group of 27 individuals that the Movement for the Emancipation of the Niger Delta (MEND) said it had freed from the pirates in mid-September and whom it had since been holding.
Nineteen hostages had already been released in early October.
The movement added however that it would hold onto two British nationals, seized at the same time on the same vessel, for as long as its leader Henry Okah, currently on trial in central Nigeria, remained in custody.
"The remaining two British nationals will be kept with us until the government of Gordon Brown stops offering training and materials to the Nigerian armed forces which has turned them into brutal animals against the civilian population in the Niger Delta and now spreading to other regions," MEND said.
The group, which claims to be fighting for a greater share of the oil revenue for the local population in southern Nigeria, said the freed hostages had been handed over to the Niger Delta Peace Committee..
MEND came to prominence in January 2006 and has since claimed responsibility for a string of violent attacks on Nigeria's oil industry and kidnappings of both local and foreign oil workers in the region.
The unrest has reduced Nigeria's oil output by more than one quarter. Production currently stands at between 1.8 and two million barrels a day against 2.6 million barrels two years ago.