A militant group in Nigeria's Niger Delta region has claimed responsibility for killing four police officers in an attack on a checkpoint.

The Movement for the Emancipation of the Niger Delta (MEND) said in a statement Friday that it carried out Thursday's attack on a marine police checkpoint in the southern state of Bayelsa. It is the second attack claimed by the group this month.

The group also said it was in contact with the abductors of three crew members of a Dutch-owned ship that authorities said was attacked by pirates Tuesday off the Nigerian coast.

It said the kidnappers have offered to transfer custody of the hostages to MEND, and said the hostages were in good health.

MEND carried out a series of attacks on foreign oil companies before most of its members signed an amnesty with the Nigerian government in 2009.

The militants claimed responsibility for an attack on an Italian oil pipeline last month, raising fears the group was re-organizing.

In its statement Friday, the group warned that it would carry out future attacks on oil industry vessels operating off the Nigerian coast. It said it would launch rockets and set on fire any “uncooperative” ship that implements anti-piracy maneuvers.