A Ukrainian captain and Greek engineer who were kidnapped from their ship by pirates off the Nigerian coast last month have been released, the Greek coast guard said on Wednesday.

Armed pirates attacked the Marshall Islands-flagged MT ALTHEA oil products tanker around 35 nautical miles off the coast of Akwa Ibom state on Dec. 16, robbing more than a dozen crew members before escaping with the two sailors.

They have been freed and they are both well,” a Greek coast guard official said, asking not to be named. It was not clear what prompted their release but security experts say ransoms are usually paid. A Nigerian Navy spokesman said he had no details on the incident.

Pirate attacks off Nigeria are on the rise as ships passing through West Africa’s Gulf of Guinea, a major commodities route, have come under threat from gangs wanting to snatch cargoes and crews.

Unlike the waters off Somalia on the east African coast, where ships now speed through with armed guards on board, there is little protection for many vessels which have to anchor off West African countries to do business. This makes them easier targets for criminal gangs who operate in Nigeria’s volatile coastal oil region and pushes up costs for shipping firms.