Brussels: The number of migrants arriving in Greece dropped 90 per cent in April, the European Union border agency said on Friday, a sign that an agreement with Turkey to control traffic between the two countries is working.

The agency, Frontex, said 2,700 people arrived in Greece from Turkey in April, most of them from Syria, Pakistan, Afghanistan and Iraq, a 90 per cent decline from March.

The tents of migrants at the Athens port of Piraeus. Photo: Thanassis Stavrakis/AP

Under the EU's agreement with Turkey, all migrants and refugees, including Syrians, who cross to Greece by boat are sent back.

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In return, the EU will take in thousands of Syrian refugees directly from Turkey and reward it with more money, visa-free travel for its citizens and faster progress in EU membership talks.

In Italy, 8,370 migrants arrived through the longer and more dangerous route from northern Africa, Frontex said. Eritreans, Egyptians and Nigerians accounted for the largest share.

There was no sign migrants were shifting from the route to Greece to the central Mediterranean route, Frontex said. The number of people arriving in Italy in April was down 13 per cent from March and down by half from April 2015.

That particular statement was contested by the Norwegian Refugee Council, an Oslo-based humanitarian agency. It cited Thursday's announcement by Italian coastguards that they had helped rescue 801 people, including many Syrians, from two boats heading from Northern Africa to Italy.

"This might be a first sign of Syrian refugees now choosing the much more dangerous route across the Mediterranean from Northern Africa to Italy, in search of protection in Europe," said Edouard Rodier, Europe director at the council.

"If this continues, the EU-Turkey deal is not only a failure, but may also result in more deaths at sea," he said.

Italy also announced on Friday it had deployed 110 more guards to keep migrants from travelling into Austria, after Vienna threatened to introduce tighter border controls that could have hurt trade.

Italian Interior Minister Angelino Alfano, speaking alongside his Austrian counterpart Wolfgang Sobotka at the Brenner Pass, said the extra guards would patrol the Alpine crossing point between the two countries.

Both Italy and Austria are members of the European Union's Schengen open-border zone, but Vienna said last month it would erect a fence at the border and slow traffic along the highway that crosses it if there was a surge in migrant arrivals this summer.

"Migrants who arrive here thinking to go to Austria will be taken to Italian shelters," Mr Alfano said.

"A plan to slow traffic would have an enormous impact on trade and travellers."

The pass is Italy's main commercial route to Germany, its top trading partner, by way of Austria.

Reuters