Pirates have kidnapped 15 crew members from a chemical tanker in the Gulf of Guinea after boarding the vessel off the coast of Benin, the ship's Dutch owners said on Friday.

The incident happened on Thursday afternoon around 210 nautical miles (389 kilometres) south of Cotonou when pirates attacked the chemical tanker Davide B, the De Poli tanker company said.

"Fifteen crew members have been taken... while six other seafarers of the vessel are safe and unhurt," said the company, based in Barendrecht, just south of the Dutch port city of Rotterdam.

The six crew members "remain on board of the ship, which is currently attended by security personnel," De Poli Shippingmanagement said.

The Maltese-registered Davide B was sailing from Riga to Lagos in Nigeria when the attack happened.

"The company's management is now greatly concerned about the well-being of the missing crew," said company spokesman Cor Radings.

"Our main priority now is to establish contact with the missing crew in order to secure their earliest and safe release," he told AFP.

He could not give details about the missing crew members, but they are believed to be Russian, Ukrainian and from the Philippines.

Kidnap attacks on ships for ransom have become common in the Gulf of Guinea which runs from Senegal to Angola, taking in the southwest coast of Nigeria.

The perpetrators are usually Nigerian pirates.

The Gulf of Guinea accounted for more 95 percent of all maritime kidnappings last year -- 130 out of 135 cases, according to the International Maritime Bureau (IMB), which monitors security at sea.