Pirates attacked an Italian ship off Nigeria and kidnapped four sailors, including three Italians, in the latest such incident off the country's oil-producing southern coast, officials said Monday.

While a spokeswoman for Italy's foreign ministry confirmed three of those abducted were Italians, the nationality of the fourth victim was not yet clear following the Sunday night attack.

"Pirates armed with guns attacked, boarded an offshore supply vessel ... and kidnapped four crew members," the International Maritime Bureau said.

There were "no injuries to crew members and (the) vessel continued passage to a safe port," it said.

The attack occurred some 40 nautical miles off of Nigeria's Bayelsa state, the IMB said. Nigerian navy spokesman Kabir Aliyu named the ship as the MV Asso Ventuno and said five vessels had been deployed in the area to patrol.

The Italian foreign ministry confirmed an attack on an Italian ship. It said Italian officials were monitoring the situation closely and were in contact with local authorities.

Such kidnappings occur regularly off Nigeria's oil-producing Niger Delta region, with hostages typically released after payment of a ransom.

A 2009 amnesty deal led to a sharp drop in unrest in the region, but criminality remains widespread.

Noel Choong, head of the IMB's piracy reporting centre in Kuala Lumpur, told AFP that attacks off Nigeria and the Gulf of Guinea have risen to 51 this year with the latest incident.

He said the dozens of incidents in the region make it one of the most dangerous areas for seafarers after Somalia.

Choong said since August 2012, there have been three kidnapping incidents off Nigeria involving 15 crewmen, including the latest incident.

Kidnappings occur both onshore and offshore in the Niger Delta.

On December 14, the elderly mother of Nigeria's finance minister was freed from kidnappers after being abducted from her home in a crime that shocked the country.

Just a week ago on December 17, four workers from South Korea's Hyundai Heavy Industries and a Nigerian were abducted by armed men while working at a construction site in Bayelsa. The five were freed unharmed on Friday night.

A gang of about seven armed men stormed into their office, shot into the air to scare people away and made off with the hostages in a waiting speed boat to a mangrove forest, the Nigerian hostage said after his release.

He said they were guarded by armed men throughout the ordeal and that their abductors made a series of telephone contacts with the company before deciding to free them.

Also on December 17, well armed pirates stormed and ransacked an oil tanker off the Niger Delta and kidnapped five Indian crew members.

In addition to kidnappings, pirates regularly attack tankers off the West African coast to steal fuel cargo for sale on the region's lucrative black market.

Such tanker hijackings have long occurred off Nigeria, but have recently spread to other countries in the region, including Benin and Togo, prompting authorities to boost naval patrols.

Nigeria is Africa's largest oil producer and the continent's most populous nation with some 160 million people. The Niger Delta region remains deeply impoverished despite its oil reserves, with corruption rampant.

Kidnappings in Nigeria's south are considered a different phenomenon than those that have occurred recently in the country's mainly Muslim north, where Islamist extremists are suspected of having abducted British, Italian, German and French hostages.

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