Twelve crew members of a Swiss merchant vessel kidnapped by pirates off the coast of Nigeria are from the Philippines, Slovenia, Ukraine, Romania, Croatia and Bosnia, the West African nation's maritime agency said on Sunday.

Massoel Shipping said its bulk carrier MV Glarus, with 19 crew, came under attack early Saturday morning as it transported bulk wheat from Nigeria's commercial capital Lagos to the southern oil hub of Port Harcourt.

During the attack, 45 nautical miles southwest of Bonny Island, "the pirate gang boarded the Glarus by means of long ladders and cut the razor wire on deck to gain access to the vessel and eventually the bridge," the company said in a statement sent to AFP.

"Having destroyed much of the vessel's communications equipment, the criminal gang departed taking 12 of the 19 crew complement hostage," it added.

Massoel spokesman Patrick Adamson told AFP it was the second largest number of people taken from a ship in the oil delta southeast of Nigeria this year.

"Pirates usually don't make contacts in the first 48 hours, so we don't have any news on that," he added.

For safety reasons, neither the identities or nationalities of the hostages would be released, he said.

More than 60 crew were reported kidnapped last year in waters off Nigeria, Africa's leading crude producer, although many attacks are not reported.

The Geneva-based shipping company said "all the appropriate authorities have been notified" and specialists have been called in to "secure the speedy and safe release of those being held."

"Families are being kept closely informed of the situation."

Switzerland's foreign ministry said it "has been informed of the attack on Glarus, a vessel sailing under a Swiss flag along the Nigerian coast."

The Swiss Maritime Navigation Office was in contact with the vessel's operator, it added.

Kidnapping for ransom is a common problem in parts of Nigeria. A number of foreigners have been kidnapped in the last few years in the southern Niger Delta region, source of most of the oil that is the mainstay of the West African nation's economy.

Sunday Umoren from the Nigerian Maritime Administration and Safety Agency (NIMASA) said seven of those abducted were from the Philippines and one from each of the other five countries.

"They are still missing," said Umoren, who heads maritime safety at NIMASA.

The number of seafarers taken hostage in the Gulf of Guineau - the region where the attack took place - rose from 52 in 2016 to 75 last year, said Jake Longworth, senior analyst at EOS Risk Group, in a report published in July.

He said that although the number of people kidnapped was similar in the two years, there was a trend for pirates to take more hostages in each attack. Longworth said 35 seafarers were kidnapped for ransom in the region in the first half of 2018.