Thirteen seafarers have been kidnapped from a product tanker in what is thought to be the furthest offshore pirate attack in the Gulf of Guinea to date.

The seafarers, Ukrainians and Russians, were aboard the 11,322-dwt Curacao Trader (built 2007) and 452 km south of Conotou when eight armed men boarded the ship and took them prisoner, maritime security consultancy Dryad Global said on Friday.

The incident was confirmed by the ship's manager, Alison Management Corp, on Saturday.

Alison said the attack occurred at 11am (1000 GMT) local time and that all but six crew were taken hostage.

"As a result, the ship is currently drifting with limited man power on board," the shipmanager said.

Assistance is being provided by the 8,097-dwt refrigerated cargo carrier Frio Chikuma (built 1998), Alison said.

Both ships are connected to the Laskaridis family.

"Alison Management Corp wishes to advise that its prime concern remains the safety and recovery of its abducted crews and no effort shall be spared to achieve their soonest possible release," the company said.

According to AIS data, the Curacao Trader left Lome on Thursday and last broadcast a signal at 5:22pm in international waters.

The ship, Dryad said, slowed to 3 knots at the time of the attack. It drifted until 3:15pm before speeding up to 10 knots.

The Curacao Trader's AIS status says it is not under command.

The ship is Liberian flagged, insured by Gard and is owned by Curacao Trader Shipping, which shares an Athens address with Laskaridis-linked Lavinia Corp.

On Thursday, the International Maritime Bureau (IMB) reported a total of 77 kidnappings in the first six months of 2020, a year-over-year increase from 75, with 49 taking place in the Gulf of Guinea.

The agency said the kidnappings were happening further from shore, with attacks happening as far as 130 nautical miles from the coastline.

The most recent attack was on 2 July, when five Chinese seafarers were taken from the 27,378-dwt cargoship Kota Budi (built 2013) offshore Benin.

Seafarers are typically held for six weeks, the IMB said.

Dryad said the number of kidnappings this year had jumped 20% from 2019 and the number of personnel kidnapped rose 47% over the same period.

In the Curacao Trader attack, it said speedboats were involved but "it remains highly likely that the perpetrators are being supported by a larger mother vessel, allowing them to sustain deep offshore operations beyond state counter piracy efforts."