According to the respective governments and a crew list seen by Reuters, one sailor was killed in the raid, an Azerbaijani citizen, while those kidnapped are from Turkey.
Pirates off Nigeria's coast kidnapped 15 sailors from a Turkish-crewed container ship in the Gulf of Guinea on Saturday in a brazen and violent attack farther from shore than usual.
According to the respective governments and a crew list seen by Reuters, one sailor was killed in the raid, an Azerbaijani citizen, while those kidnapped are from Turkey.
Accounts from the crew, family members and security sources described a sophisticated and well-orchestrated attack on Saturday in which armed pirates boarded the ship and breached its protective citadel, possibly with explosives.
Three sailors remain on the Mozart, which was receiving assistance in Gabonese waters off central Africa by Sunday evening.
"The ship is in our waters and our sailors are assisting a few nautical miles from Port Gentil," said Gabon's presidency spokesman Jessye Ella Ekogha, without providing further detail.
The Liberian-flagged vessel was headed to Cape Town from Lagos when it was attacked 160 kilometres (100 miles) off Sao Tome island on Saturday, maritime reports showed.
Turkish President Tayyip Erdogan's office said that he was orchestrating officials in the "rescue of kidnapped ship personnel" on Sunday.
Erdogan spoke twice by phone with the ship's fourth captain, Furkan Yaren, who remained aboard after the attack, his office said.
State-run Anadolu agency cited Yaren as saying he had been "cruising blindly" toward Gabon with damage to the ship's controls and only the radar working. The pirates beat crew members, and left him with an injured leg while another still aboard the ship had shrapnel wounds, he said.
Turkish media cited Istanbul-based Boden company, which provides technical management services for the vessel, saying the vessel operators were abducted at gunpoint. Boden was not immediately available.
Ambrey, a security company, said four armed men boarded the Mozart and entered the citadel - where the crew are advised to hide in any attack - through a hatch or the door.
Edward Yeibo, a Nigerian Navy commander, said he was not aware of the attack and seeking details. The Lagos naval command office and a spokesman for Nigeria's maritime regulator were not immediately available.
According to an International Maritime Bureau report, pirates in the Gulf, which borders more than a dozen countries, kidnapped 130 sailors in 22 incidents last year, accounting for all but five of those seized worldwide.
The attack on the Mozart could raise international pressure on Nigeria to do more to protect shippers, which have called for stricter action in recent weeks, analysts said.
"The fact that someone died, the number of people taken and the apparent use of explosives to breach the ship's citadel means it is a potential game-changer," said David Johnson, CEO of the UK-based EOS Risk Group.
"It's quite sophisticated and if pirates have decided to use munitions it's a big move," he said. There is "no doubt" those kidnapped will be taken back to Nigeria's Delta and Turkey will have little hope stopping it, he added.
Turkey's foreign ministry said the pirates had not contacted Ankara.
Seyit Kaya, brother of the ship's kidnapped 42-year-old captain Mustafa Kaya, a father of two, said in an interview he awaited details from the ship's owner on any possible ransom.
"Since that area is where many attacks occur, they take cautions against pirates," said Kaya, who is also a sailor.