Amnesty International and Human Rights Watch (HRW) on Thursday called for a “credible” probe into a migrant shipwreck off Greece in June in which hundreds died, saying that contrasting accounts by the Greek coastguard and survivors “were extremely concerning”.

The overcrowded fishing trawler said to be carrying between 400-750 people from Pakistan, Syria and Egypt sank in international waters off Greece on its way to Italy from Libya.

Some 104 men survived and authorities recovered only 82 bodies.

Survivors have recounted a doomed attempt by the Greek coast guard to tow the trawler that caused the vessel to capsize, according to interviews and evidence seen by Reuters.

The Greek coastguard and government have said no attempt was made to tow the boat and that it overturned when the coastguard was about 70 metres away.

Greek judicial authorities have launched an investigation into the causes of the disaster which could take more than a year. The acts of the coast guard are also under investigation.

In a joint statement, Amnesty and HRW said they interviewed 19 survivors, four relatives of the missing as well as nongovernmental organisations, UN agencies and representatives of the Greek coast guard and police during a visit to Greece between July 4-13.

“Survivors interviewed by Amnesty International and Human Rights Watch consistently stated that the Hellenic Coast Guard vessel dispatched to the scene attached a rope to the Adriana and started towing, causing it to sway and then capsize,” they said.

Judith Sunderland, Associate Europe and Central Asia Director at HRW, said the disparities between survivors’ accounts and the authorities’ version were “extremely concerning.”

Both groups called for “a full and credible investigation into the shipwreck … to clarify any responsibility for both the sinking of the ship and delays or shortcomings in the rescue efforts that may have contributed to the appalling loss of life.”

The EU rights watchdog last month also announced a probe into the sinking and whether Frontex, the bloc’s border agency, fulfilled its rescue duties.