Athens - Unidentified gunmen have staged a series of new sabotage attacks on boats carrying migrants cross the Aegean Sea, Human Rights Watch said on Thursday.
Quoting witnesses, the New York-based rights group said there had been eight incidents in which gunmen "intercepted and disabled the boats carrying asylum seekers and migrants from Turkey toward the Greek islands".
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The most recent incidents were on October 7 and 9.
According to a 17-year-old Afghan called Ali, a speedboat with five men armed with handguns had rammed their rubber dinghy on October 9.
"At first when they approached, we thought they had come to help us," he told HRW.
"But by the way they acted, we realised they hadn't come to help. They were so aggressive. They didn't come on board our boat, but they took our boat's engine and then sped away," he said.
He said the men attacked three other boats in quick succession before speeding off toward the Greek coast
"They spoke a language we didn't know, but it definitely was not Turkish, as we Afghans can understand a bit of Turkish," he said.
Similar allegations were made by both migrants and rights groups during the summer.
The latest attacks took place near the island of Lesbos, HRW said.
A Greek coastguard source said the claims were under investigation but despite searches, they had not been able to locate the alleged perpetrators.
In August, the coastguard arrested three men on the island of Samos suspected of preying on migrants seeking to cross over from Turkey.
They were dressed like members of the Greek coastguard and wore hoods, the coastguard said.
At the time, several refugees from Syria and Iraq on the island of Kos told AFP they had been attacked by masked gunmen at sea, with some claiming the assailants stole their fuel and even their motors.
Some accused the Greek coastguard of assaulting them.
Over half a million migrants and refugees have arrived in Greece, most of them fleeing violence in wartorn Syria.
The influx has shown little sign of letting up and has threatened to overwhelm the authorities, particularly on the Greek islands.
On Wednesday, the International Rescue Committee said 16,000 people were stranded on the island of Lesbos owing to a registration bottleneck.
"Food is in short supply, as is access to water, toilets or suitable shelter," the IRC said.
Greece's interior ministry on Thursday said it needed an additional 330 million euros ($375 million) to upgrade registration centres on the islands.
It said the response of EU states who had been expected to provide additional border staff fell far below requirements.
Out of 775 staff requested by EU border agency Frontex, only 48 had been pledged by six out of the 28 member states, the ministry said.
"We are making every effort to carry out our responsibilities," the ministry said in a note.
"Our response time can only be equivalent to the response time of the rest of Europe," it said.