Nigerian sources said the master, chief officer, chief engineer, second engineer and third engineer were seized, along with cash, from the 2,578–teu City of Xiamen (built 2008) on 26 April.
One is said to be Russian and the others Polish, with a woman possibly among the hostages.
The ship is now safely off Malabo in Equatorial Guinea.
Owner Sunship confirmed the incident and said: “Noinjuries were reported among the remaining crew members who continued with thevessel to a safe port. Sunship is in contact with the families of those beingheld and those on board.
"All the appropriate authorities have been contacted, including the Germanpolice and flag state [Antigua & Barbuda].
“Sunship is doing utmost to ensure the safe return of its seafarers but willnot be providing any operational details
which could jeopardise the safety of those being held.”
The attack involved 14 armed men in two boats 100 miles south of Brass.
One of the vessels involved is thought to be the crew boat Utai 8, hijacked last week.
One report said the men on City of Xiamen took shelter in the citadel, but this was breached.
There were 13 men on board, including a Russian, 11 Sri Lankans and a Myanmarese.
The incident follows a spate of attacks in the area in the last two weeks.
Four crew were seized last week from the 1,740-teu Hansa Marburg (built 2007), 130 miles south-west of Malabo.
Two other boxships have been attacked in the last week and two oil workers were kidnapped from a rig in the Sengana field.
Earlier in April, Euronav's 159,000-dwt suezmax Cap Theodora (built 2008) was attacked.
Maritime security consultant Bergen Risk Solutions is advising ships to stay at least 200 miles from the coast in the Niger Delta area.
It described another mothership being used in the attacks as a trawler with a red and white superstructure.
