Migrants head to Switzerland amid Austria border crackdown

"The number of African illegal stayers who primarily entered the EU through the Central Mediterranean route significantly decreased in Austria, while at the same time grew in Switzerland," said Frontex.

Fewer people are also entering Germany from Austria. At the same time, more detections have since been made at the Swiss border.

Austria imposed additional checks at its border with Italy last summer and threatened to close the frontier at the Brenner Pass, a major gateway. People caught at the border are turned back to Italy.

The move appears to have diverted the flow elsewhere as people seek alternative routes in an effort to get to their final destinations further north.

France noted a 156 percent detection increase in 2016 compared with the previous year. The French also noted a large spike in people now attempting to enter through Spain and Portugal.

Data kept from public

Frontex said the reinstatement of controls at internal borders had simply "led to a diversification in routes and modi operandi."

Austria, Denmark, Germany, Sweden and non-EU member state Norway have all introduced temporary border controls. The data that justifies those checks is not being shared with the public.

The issue has riled some MEPs who are demanding that the European Commission or the EU Council, representing member states, release the information.

"How can we assess whether or not these controls are needed if we do not have that information?" centre-right Portuguese MEP Carlos Coelho asked the EU Commission in a discussion on Schengen last month.

"How many controls are then in these countries, how many illegal immigrants did we stop, what time type of controls are being done, targeted, what is the economic impact of these controls?" he said.

Western Balkans to Italy

A large number people who managed to travel through the Western Balkans and enter Hungary have also headed towards Italy.

Italy last year reported some 10,000 detections from people who had already been fingerprinted in Hungary. The vast majority were from Afghanistan and Pakistan.

Frontex said the figures "clearly point to a smaller branch of the Western Balkans route leading to Italy".