BRUSSELS -- The European Union's border agency is boosting its operations in the Mediterranean Sea near Greece as increasing numbers of migrants try to enter the EU from Turkey, its chief said Thursday.

"We are worried about the situation in Greece and that's why we will upgrade our action there and our support to the Greek authorities," Frontex Executive Director Fabrice Leggeri told The Associated Press in an interview.

More than 10,000 people have been plucked from the central Mediterranean in recent weeks attempting to enter Europe from Libya on overcrowded, unseaworthy boats. The International Organization for Migration estimates that nearly 1,830 migrants have died crossing the Mediterranean this year compared to 207 in the same period last year.

"There is a shift from the central Mediterranean to the eastern Mediterranean" as more migrants leave Turkey by sea and land, Leggeri said. "They are moving very quickly, so we have to be flexible."

Earlier Thursday, police in northern Greece said 93 Syrian migrants were released from a locked cargo train carriage after being tricked by smugglers into travelling in the wrong direction.

The number of Syrians reaching Greece has surged in the past year as the civil war in their country continues into a fifth year. Many make the short hop from Turkey's southwest coast toward the island of Lesbos 9 kilometres (5 miles) away in inflatable rafts. From there, they often try to board ferries and move further north into Europe.

But others choose to travel by land through western Turkey and across the Greek and Bulgarian borders, where the Frontex presence is also being stepped up.

Leggeri also said ships and planes promised for Frontex's Triton Operation to patrol waters off Italy had begun arriving, including British, French and Portuguese vessels. The border agency plans to significantly boost its presence on the Mediterranean from June to September, the high season for migrant crossings.

"We have already increased by 50 per cent in terms of assets deployed and by June we will be able until September to triple the number of boats," he said.

Triton has no mandate to carry out search-and-rescue work but has rescued thousands by putting its ships in places where they are required by international law to respond to emergency calls.

Triton currently has 10 patrol vessels, three offshore patrol ships, three aircraft and two helicopters at its disposal.