Pirates took six hostages during an attack on a commercial container ship in international waters in the Gulf of Guinea on Monday, a spokesperson for the Danish military said on Tuesday.

A Danish frigate operating in the area amid heightened security risks from pirates sent a helicopter on Monday to help rescue the container ship that was under attack from pirates in a skiff near Bioko Island, a territory belonging to Equatorial Guinea.

The helicopter followed the skiff with the six hostages on board as it sailed towards the Niger Delta, but stopped when it reached Nigerian national waters, where it is not allowed to operate, the spokesperson told Reuters.

The container ship was sailing under a Liberian flag, the spokesperson said, but the name or operator of the vessel were not immediately clear.

Denmark deployed the frigate Esbern Snare to the Gulf of Guinea in October. Last month, the frigate crew shot and killed four pirates in waters south of Nigeria and still holds four other pirates who were captured in the operation on board the frigate. Danish officials have been in talks with governments in the region to transfer the four pirates off the frigate.