Gunmen attacked a barge belonging to an oil services company off the coast of Nigeria on Saturday, killing two Nigerian sailors and kidnapping four foreigners, navy officials said.

The suspected sea pirates stormed the vessel in the Gulf of Guinea, an area that has seen a sharp spike in the number of reported marine attacks over the last six months.

A spokeswoman for Sea Trucks Group, which provides support vessels to oil companies operating in Nigeria, told AFP that one of the company's ships came under fire and that four of the firm's employees were taken in the raid.

"At this time Sea Trucks Group is making every effort to ascertain the whereabouts of its personnel," the spokeswoman, Corrie van Kessel, told AFP.

She said that Sea Trucks Group is heavily involved in the oil and gas sector in the Niger Delta.

Nigeria's navy spokesman Commodore Kabir Aliyu said during the attack "four expatriates are reported to have been kidnapped from the vessel; two sailors were killed."

He said those kidnapped were from Indonesia, Iran, Malaysia and Thailand.

"We have not made any arrest yet. We are still trying to ascertain the true identities of the attackers and their motive," he told AFP.

"We are still searching. We have deployed a boat and a helicopter to the area where the the attack took place," he said, adding: "We have not established any contact with the attackers or the foreigners abducted."

Aliyu said six naval personnel were stationed on board the Sea Trucks Group vessel following a security request from the company.

The attack, which also left two other seamen injured, took place at roughly 0100 on Saturday, 35 nautical miles off Nigeria's oil-rich Niger Delta coastal area, the navy and company said.

The volatile area was for years crippled by armed insurgency, largely made up of militants from the Delta who claimed that the region's people were not benefitting from its vast oil wealth, while crude production devastated the environment.

Armed groups in the Delta were notorious for kidnapping oil workers, especially foreigners.

A 2009 amnesty deal greatly reduced the unrest, but sporadic incidents have continued to occur including robberies and, most prominently, piracy.

The International Maritime Bureau (IMB) said in a report released last month that there had been 32 piracy incidents recorded in the Gulf of Guinea in the first half of 2012, up from the 25 attacks in 2011.

Many of the raids have involved "high levels of violence," kidnappings, and were increasingly occurring further offshore, the report added.

Nigeria and neighbouring Benin launched joint patrols last year to address the problem.

An IMB official said attacks in the area have been under-reported for several years.

On July 27, suspected sea pirates attacked a vessel transporting workers for Italian firm Agip in the Delta's Bayelsa state, leaving at least one person dead.

Van Kessel said that two of the company's vessels came under attack, although the navy insisted only one ship was involved.

Sea Trucks Group, which also operates in Australia and East Asia, was founded as a Nigerian firm in 1977 before expanding and currently has a "corporate support office" in the Netherlands, according to its website.

Years of unrest in the Delta had curbed oil production in Nigeria, Africa's top oil producer and the world's eighth largest, but output has recovered since the amnesty.

On Friday, Nigeria said oil production had hit its highest level ever, reaching 2.7 million barrels per day.