The Greek coastguard has staunchly defended its response to a maritime disaster involving a migrant ship that sank off the southern coast of the country. Coastguard spokesman Nikos Alexiou said that private vessels as well as those belonging to the service branch had monitored the ill-fated vessel in international waters before it sank. He further denied reports from survivors alleging that a patrol boat had tried to tow the fishing boat. “There was no effort to tug the boat,” Mr Alexiou told state broadcaster ERT. A judicial investigation into the causes of the sinking is under way. Patrol boats and a helicopter spent a third day scouring the area of the Mediterranean Sea where the overcrowded fishing vessel capsized and sank early on Wednesday. The International Organisation for Migration estimates that as many as 500 passengers may have perished in the incident. The vessel was en route from Libya to Italy. The Greek coastguard pledged to continue the search and rescue operation beyond the standard 72 hours. To date, rescuers have pulled 104 survivors from the water and recovered 78 bodies, but no further people have been located since late on Wednesday. The EU's continuing struggle to establish a comprehensive migration and asylum policy has been thrown into the spotlight by the tragedy. The rescue operation has fuelled political controversy in Greece as the country prepares for a general election on June 25. “This could be the second deadliest shipwreck we have ever recorded after the tragic shipwreck of April 2015 en route to Italy,” said the UN migration agency's Missing Migrants Project. The Greek coastguard and government officials claim that patrol boats and nearby cargo ships were shadowing the fishing boat since Tuesday afternoon, after it was detected by a surveillance plane from Europe's Frontex agency. They note that the boat stopped briefly to take on food and water from a Maltese-flagged ship. However, a person on board, speaking English via a satellite phone, insisted that no further assistance was needed and that those on board wanted to continue their journey to Italy. “From 1230 GMT to 1800 GMT, the merchant marine operations room was in repeated contact with the fishing boat,” the coastguard said. “They steadily repeated that they wished to sail to Italy and did not want any contribution from Greece.” By 2240 GMT, the boat notified Athens of engine failure and it stopped moving. The patrol boat nearby “immediately tried to approach the trawler to determine the problem”, according to the coastguard. A mere 24 minutes later, the Greek patrol boat captain reported that the boat had capsized. It sank within 15 minutes, at 2.19am Greece time. Most of the survivors were relocated from a storage hangar at the southern port of Kalamata to migrant shelters near Athens. Nine people were arrested and face several charges, including negligent manslaughter and human trafficking. The suspects are expected to appear in court in the coming days. Meanwhile, officials at a state-run morgue outside Athens began the process of identifying victims, gathering DNA samples and photographing faces. Mr Alexiou, citing survivor accounts, said passengers in the hold of the fishing boat included women and children, but the exact number of missing remains unclear. Despite the tragedy, a glimmer of hope emerged as a Syrian teenager who survived the shipwreck was reunited with his elder brother on Friday. The teenager was among 71 survivors transferred to the migrant camp of Malakasa near Athens. The crisis underlines the grave risks migrants take to escape war and poverty. The details surrounding the vessel's sinking while being shadowed by the Greek coastguard remain unclear. Advocacy group Alarm Phone alleges it alerted Greek authorities and aid agencies hours before the disaster unfolded. In the wake of this tragedy, the International Organisation for Migration and the UN refugee agency have emphasised the urgent need to address gaps in search and rescue rules. The UN's migration and refugee agencies issued a joint statement emphasising timely maritime search and rescues as “a legal and humanitarian imperative”. They called for “urgent and decisive action to prevent further deaths at sea”. Adriana Tidona of Amnesty International said: “The Greek government had specific responsibilities towards every passenger on the vessel, which was clearly in distress. “This is a tragedy of unimaginable proportions, all the more so because it was entirely preventable”. Pope Francis and UN Secretary General Antonio Guterres also weighed in on the issue, with both urging European governments to do more to protect people risking their lives in search of better ones. The EU's executive commission disclosed that the 27-nation bloc is close to an agreement regarding member countries sharing responsibility for caring for migrants and refugees who undertake the perilous journey across the Mediterranean. Simultaneously, Greece and other southern EU nations, which are typically the first destinations for Europe-bound asylum seekers travelling by sea, have toughened border protection measures in recent years.