Violence against foreign oil firms and their employees has increased in the last year and seven hostages remain as captives of various different armed groups in the Niger Delta.
The remaining hostages are two Italians, one Lebanese, one American, two Filipinos and one Frenchman.
The kidnappers of the 24 Filipinos said they seized the Baco-Liner 2 because it was "suspected to have been conveying arms and ammunition imported by top politicians in the country, to destabilise the 2007 general elections in the region".
Elections in April should mark Nigeria's first democratic transition from one civilian government to the next.
The Baco-Liner spokesman said there were explosives on board the cargo ship but these were destined for oil companies that needed them for drilling and had nothing to do with politics.
"We are not engaged in any smuggling whatsoever," said the Baco-Liner spokesman.
The kidnappers said they were from the Movement for the Emancipation of the Niger Delta (MEND), But a longtime MEND spokesman denied the group was behind the abduction of the 24 Filipinos, saying a separate group had been responsible.
The kidnappers said they would take "further actions" unless Nigerian authorities met a series of demands, including the release of two jailed leaders from the delta, the payment of compensation to local villages for oil spills and the demilitarisation of the region.
MEND was behind a series of attacks a year ago in the western delta which have cut a fifth of the country's production capacity.
Nigera is Africa's largest oil producer, with all its production facilities located in the Niger Delta.
But most residents of the region live without clean water, electricity, roads or functional clinics and this fuels resentment towards the multibillion-dollar oil industry.