A 10-year-old Afghan boy has been crushed to death on board a packed boat carrying at least 66 people aiming to reach the Greek island of Lesbos, according to Greece's news agency ANA.

Scenes of panic broke out on Saturday, when the refugees saw a patrol boat from European border agency Frontex approaching their flimsy vessel, in fear they would be sent back to Turkey, said ANA.

The Greek coastguard confirmed to Al Jazeera the age of the boy but did not comment on his nationality.

The coastguard spokesman said a Frontex boat, which was carrying out a routine patrol operation east of the island, identified the vessel travelling towards the island.

The young boy was recovered unconscious from the dinghy and then transferred to Vostanio hospital in Mytilene, the capital of Lesbos, where he was pronounced dead, according to the coastguard.

He was reportedly on the boat with his parents and two younger sisters.

His mother tried to jump into the sea after finding out about her son's death, said ANA. She was rescued by Greek coastguard officers.

Greek authorities told Al Jazeera they had opened an investigation into the incident and the boy's cause of death.

'World's deadliest border'

In March 2016, the European Union (EU) and Turkey reached a deal to stem the flow of refugees from Turkey to the continent, while countries across the Balkans sealed their borders.

Those measures have led to a build-up of more than 60,000 refugees and migrants in Greece, which is an EU member.

But despite the agreement, people desperate to reach Europe have continued arriving on Greece's islands, with numbers growing over the past few months.

The death of the 10-year-old boy comes a day after the United Nations' migration agency said more than 33,000 people have died at sea trying to reach Europe since 2000, making the Mediterranean "by far the world's deadliest border".

Professor Philippe Fargues, of the European University Institute in Florence, author of the International Organization for Migration's (IOM) report, said the figures probably underestimated the actual scale of the human tragedy.

So far this year, some 161,000 migrants and refugees have arrived in Europe by sea, according to IOM figures.

About 75 percent of them landed in Italy, with the rest in Greece, Cyprus and Spain.

Nearly 3,000 others are dead or missing, said the IOM.