Pirates have stormed an oil supply vessel off southern Nigeria and kidnapped two American crew members, Nigeria's navy and US defence officials said Thursday, the latest unrest in a region described as a piracy hotspot.

A US-flagged C-Retriever owned by American oil servicing company Edison Chouest Offshore was attacked on Wednesday off the city of Brass, Nigeria's navy spokesman Kabir Aliyu told AFP.

The ship's chief engineer and captain, both US citizens, were kidnapped in the attack, said US defence officials who requested anonymity and AKE, a private security firm which closely tracks the region.

The US Navy and the Marine Corps had not yet received orders to intervene, two US defence officials told AFP.

Aliyu said the navy had launched a search and rescue mission, but could not discuss the details including whether any foreign navies were yet involved.

Edison Chouest Offshore did not reply to emails or calls seeking comment.

This was the first reported kidnapping of US nationals around the oil-producing Niger Delta region in at least two years, said an official at the AKE office in Lagos, who requested anonymity.

The kidnapping of foreigners working in the oil sector was once a common occurence in the delta.

Abductions declined dramatically after a 2009 amnesty deal with rebels in the region, but they have spiked again in recent months.

The International Maritime Bureau (IMB), which is funded by ship owners, recorded 31 pirate attacks in the first half of 2013 in the Gulf of Guinea, which includes the waters off Nigeria, Togo, Benin and parts of Ghana.

The IMB noted a surge in kidnappings, but precise statistics are hard to establish as the reporting of many incidents is conflicting.

Oil servicing vessels have been among the most frequent targets for pirates.

Often the attackers are aiming to steal crude oil being shipped from the Niger Delta to world refineries, but sailors of varying nationality have frequently been taken hostage.

Hostages have typically been released days or weeks after their abduction. Most analysts say that ransoms are paid in such cases, but the companies involved and Nigerian officials rarely comment on payments to kidnappers.

In a report released last month, the Risk Intelligence security firm said pirates in the gulf have increasingly sought to rob international vessels over the last two years.

There are signs that the region's pirates have become bolder and developed more sophisticated attack methods, the report said.

The Lagos-based AKE official, citing recent attack patterns, said the raid on the American-owned C-Retriever was likely carried out by gunmen on board two or three speed boats, with four attackers on board each boat.

Nigeria does not allow vessels operating in its waters to have private armed security agents on board.

Companies seeking armed protection against pirates must arrange to carry Nigerian naval personnel, a difficult and often unreliable process.

On Tuesday, pirates shot dead two Nigerian officers who were escorting oil workers to a field off Bayelsa state, the same area where the Americans were abducted.

The US navy, along with forces from Britain, Spain and The Netherlands last week conducted a joint training exercise with Nigeria's navy to curb piracy in the region.

Nigeria is Africa's top oil producer, generating some two million barrels per day from onshore and deepwater fields in the Niger Delta, which falls along the Gulf of Guinea.

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