Germany is to build more detention centres on its border with Austria, in the hope of returning migrants who are claiming asylum to the first EU state they entered.

The new border regime is part of the compromise Chancellor Angela Merkel's CDU party made with its coalition partner the CSU party and its leader, the interior minister Horst Seehoffer to stay in power.

Agreements will be sought with other EU states on a procedure for returning asylum seekers, but a deal with Germany's neighbour Austria and its anti-immigrant government may prove difficult to achieve.

This year between January and May 4600 illegal entries were registered on the German side of the border and roughly half were sent back to Austria. Those entries include bona fide refugees fleeing persecution from countries such as Syria, as well as other migrants who are seeking a better quality of life.

The migrant flow has declined along the Western Balkan route into the EU, but up until now the Germany Austria border has not been tightly controlled because they're both EU states.

And it begs the question of how many migrants have evaded detection altogether.

At the moment most of them come from Nigeria, followed by Afghanistan, Serbia, Albania and Pakistan.

But the countries that produce migrants, the routes they follow and the amount of people who choose to migrate vary as conflicts erupt, poverty worsens and borders around the world are tightened.

At the moment the UN refugee agency says the top five refugee producing states are Somalia, South Sudan, Syria, Afghanistan and Myanmar.