Athens (AFP) - Migrants trying to sail from Turkey to Greece are increasingly reporting being attacked by gunmen trying to prevent them from reaching Europe, according to multiple sources.

The reports came as the UN's refugee agency on Friday said Greece's infrastructure was falling "far short" of the needs of the 109,000 migrants who have arrived in the country this year, fleeing war, poverty and persecution.

An international NGO, which did not wish to be named, has launched an investigation after several witnesses reported being attacked en route from Turkey to Greece, with some saying they were robbed and others saying the motors from their boats were stolen.

The NGO said it suspected the involvement of "mafia gangs", while also noting that the Greek coastguard has previously engaged in activities designed to deter migrants from trying to make their way to Greek shores. "But we have been assured that this has finished," the NGO said.

Reports of attacks on migrants were particularly common around the Greek island of Lesbos in the east Aegean Sea, it added.

Nawal Soufi, and Italian activist who has handled numerous distress calls from Syrian refugees at sea, said she had heard of around 20 boats whose passengers had been attacked in recent months.

"The migrants speak of commandos -- some say they (were confronted by EU border agency) Frontex, others by the Turkish or Greek coastguard -- others by militias or pirates," Soufi told journalists, adding that such cases have rocketed in recent months.

Mubarak, a 30-year-old Syrian who set sail from Turkey to Greece in May, said his boat was stopped by what he believed was a Turkish navy vessel, which fired water jets in a bid to scare them away.

Men then tried to take the boat's engine, but after more than an hour's fighting the migrants managed to make it to Lesbos, said Mubarak, who has since claimed asylum in Germany.

A mobile phone video of the skirmish appears to show a military boat and silhouettes of the attackers, against the sound of the boat's panicking passengers.

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A Syrian who left Turkey on Tuesday night told Soufi how a navy boat had approached his vessel and hooded men, who carried M16 rifles and spoke a language she did not understand, came onboard.

They carried out a heavy-handed search of passengers, tearing their clothes and underwear to find hidden cash, and throwing the contents of their bags overboard.

The attackers abandoned the passengers after stealing their engine. The group was rescued by the Turkish coastguard, along with another group who had suffered a similar attack.

Efi Latsoudi of the NGO "Village All Together", based in Lesbos, said the attacks may be aimed at dissuading migrants from making the trip, tactics that have been common in Greece in the past.

"Amnesty International and other human rights groups have repeatedly denounced the poor treatment of migrants in Greece, who are beaten and intimidated in a bid to convince them to return to Turkey," she told AFP.

The UN's refugee agency, the UNHCR, on Friday urged the European Union to provide "urgent and robust" support to Greece to help it deal with the crisis.