Pirates have kidnapped four sailors after attacking their vessel off Nigeria's coast, the International Maritime Bureau (IMB) has said.
"The Nigerian Navy in collaboration with other agencies have intensified search and rescue efforts of the four kidnapped crew members of MV Asso Ventuno and apprehension of the pirates"
- Nigeria navy spokesperson
The hostages were foreigners - three of them Italian, AP news agency reports.
The Italian-registered vessel, named by the navy as the MV Asso Ventuno, was boarded on Sunday off the coast of Nigeria's Bayelsa state, the IMB said on Monday.
There were no injuries to other crew members and the pirates released the ship, which continued to a safe port, IMB said.
"Five vessels have been deployed for patrol duties off Brass area," a spokesperson for Nigeria's navy said.
"The Nigerian Navy in collaboration with other agencies have intensified search and rescue efforts of the four kidnapped crew members of MV Asso Ventuno and apprehension of the pirates," the spokesperson added.
The abduction of the three Italians was also confirmed by Italy's Foreign Minister Giulio Terzi.
No comment was immediately available from the ship's owner, a Naples-based oil services company.
Rise in kidnappings
Piracy and kidnapping in Nigeria's oil-producing Delta and offshore are common and the Gulf of Guinea is second only to the waters off Somalia for the risk of pirate attacks, which drives up costs for oil and shipping firms.
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 the AFP news agency 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.
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.
Last week, pirates looted an oil tanker off Nigeria and abducted five crew, the ship's operator said.
Four South Koreans and two Nigerians were snatched in Bayelsa earlier this month but were later released.
Nigerian criminal gangs operating in the Niger Delta usually free captives after ransom is paid.
Gunmen abducted two Lebanese men working for Nigerian construction company Setraco in Delta state this month, and killed a soldier who had been protecting them. They are still missing.
The 83-year-old mother of Finance Minister Ngozi Okonjo-Iweala was kidnapped on December 9 in Delta state but freed five days later after a military search.
In the north of Nigeria, where rebel fighters operate, another form of kidnapping of foreigners has emerged this year.
A Nigerian rebel group said on Sunday it was behind the kidnapping of a French national last week, citing France's ban on full face veils and its support for military action in Mali.
Western governments are increasingly concerned that fighters in northern Nigeria are linking up with outside groups, including al-Qaeda's North African wing.