The 16 Indian crew members held on board a seized oil tanker must be investigated fairly and transparently by Nigerian authorities over oil theft allegations after suffering months of stress in detention, two Indian ship managers said on Monday.

The two associations warned that the mental and physical health of the men had declined while they had been held for more than three months and urged the authorities not to rush to “any hasty summary judgement not backed by a proper inquiry”.

The 300,000-dwt Heroic Idun (built 2020) was on a sub-charter to BP when it was detained by Equatorial Guinea’s Navy in August at the request of Nigeria following claims that it was trying to lift oil without authority from a terminal in the Akpo oilfield off Port Harcourt.

The ship’s owner, Idun Maritime, protection and indemnity insurer Gard, ship manager OSM and BP have all said the vessel had been cleared to load when it was detained and have accused Nigeria of illegally taking the men from Equatorial Guinea to stand trial.

They say the 16 Indians and 10 other crew members have done nothing wrong and the Nigerian authorities are using the men to demonstrate they are getting tough on oil theft, which is costing the country billions of dollars every year.

“The two associations hope … that the detained seafarers will not be subjected to any undue stress nor duress, and that any enquiry in this regard will be fair and transparent,” said the Foreign Ship-owners Representatives & Ship Managers Association and the Maritime Association of Shipowners Shipmanagers & Agents, which together represent 75% of Indian crew on foreign-flag ships.

“An appeal has been made to appropriate ministries in India to intervene on behalf of the seafarers and hasten the process of logical culmination of this unfortunate episode,” the associations said in a joint statement.

Some of the seafarers were kept on board while others were detained in Equatorial Guinea and questioned for up to 14 hours, according to the crew’s representatives.

The master and senior crew members have made a series of social media appeals for help from the Indian government. “We have to go home, you have to help us here,” said the chief officer.

The Heroic Idun was returned to Nigeria on 11 November and the crew were taken before a court last week. They are due to stand trial in January and face long jail terms if found guilty. They have been bailed to remain on the ship.