ONITSHA, Nigeria (Reuters) - 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.

They were abducted on Saturday from the vessel that was traveling between the cities of Lagos and Port Harcourt. The ship’s operator said the attack happened around 45 nautical miles southwest of Bonny Island.

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.