A militant group in Nigeria's oil-rich delta on Friday claimed responsibility for killing four police officers this week in the group's second assault in about a month. Kidnappers of three foreign ship crew members have also contacted the group, it said.

Four police officers were on a boat patrolling the Nembe river in Bayelsa state on Thursday when gunmen shot them, said Bayelsa State police spokesman Eguavoen Emokpae. The Movement for the Emancipation of the Niger Delta, known as MEND, said they were responsible for the attack in an email to journalists Friday.

The police attack comes about a month after MEND claimed responsibility for an attack on an oil pipeline in the area, run by Italian firm Eni SpA.

Before the recent pipeline attack, the group had been relatively quiet. The two attacks raise fears that MEND, which once crippled Nigeria's oil industry with a wave of attacks targeting foreign oil companies, could be remobilizing. The statement also raises concerns about militants' ties with pirates.

The group said in the statement that the kidnappers of three foreign crew members have offered to hand over their hostages to MEND. It said the hostages were in good health.

The International Maritime Bureau had reported that a captain and chief engineer were kidnapped off a Dutch-owned vessel anchored in the city of Port Harcourt during an attack by pirates Tuesday. It said a crew member was also missing.

The Curacao-flagged vessel laden with refrigerated cargo had 14 crew members from Russia, Ukraine and the Philippines, said Noel Choong, who heads the bureau's piracy reporting center in Kuala Lumpur. Choong did not give the kidnapped crew members' nationalities.

The MEND statement named the hostages and said the captain and chief engineer are Russian, while the crew member is from The Philippines.

"We are considering this offer (to take over the hostages) as these men were not captured from a vessel related to the Nigerian oil industry," the statement read. It did not say what was required for the hostages to be freed.

"We are studying the statement," said Lt. Col. Timothy Antigha, spokesman for a special taskforce dealing with the threat of militancy in the oil-rich region. "We've learned to take these claims with a pinch of salt."

Oil militancy started in 2006 in Nigeria's oil-rich southern delta, where foreign firms have pumped oil out of the country for more than 50 years. That violence ebbed in 2009 with a government-sponsored amnesty program promising ex-fighters monthly payments and job training. However, few in the delta have seen the promised benefits and scattered kidnappings and attacks continue, though at much smaller scale than they used to occur.

Meanwhile, piracy has been on the rise in Nigerian and neighboring waters.

Over the last year, piracy in the Gulf of Guinea -- which follows Africa's southward curve from Liberia to Gabon -- has escalated from low-level armed robberies to hijackings and cargo thefts.

About two weeks ago, pirates killed the captain of another cargo ship off Nigeria. The maritime bureau said the ship's chief engineer tried to escape and died of a fall during the raid, and was not shot dead as it reported earlier.

In August, London-based Lloyd's Market Association, an umbrella group of insurers, listed Nigeria, neighboring Benin and nearby waters in the same risk category as Somalia in East Africa, where two decades of war and anarchy have allowed piracy to flourish. Analysts, however, have said that the type of crime in the two waters is very different, with Nigerian pirates increasingly targeting vessels carrying refined petroleum products in the oil-rich region.

Nigeria is Africa's biggest oil producer and a top exporter to the U.S.