More migrants trying to make their way into Europe are arriving in Italy than Greece for the first time in a year, according to the latest UN figures.

While 9,149 migrants arrived to the shores of Italy in April, Greece received 3,462 people, figures of the UN's refugee agency show.

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In March, before the EU-Turkey deal came into force, under which Ankara pledged to take back migrants from Greece, 26,971 people arrived to Greece, while 9,676 arrived to Italy.

Arrivals through the Aegean Sea have dropped after the EU-Turkey deal kicked in.

Last year in April, 13,556 migrants made their way to Greece, while 16,063 people made their way to Italy from Africa in the hope of seeking asylum in the EU.

But migrants arriving in Italy are not coming from Syria, Iraq and Afghanistan, the three major groups trying their luck through Greece.

Asylum seekers making their way through the Mediterranean are primarily from Nigeria, Gambia, Somalia and other Sub-Saharan African nations.

Concerns that migrants will increasingly take the Italian route have prompted fears of another mass influx, leading Austria to plan to restrict access through the its Brenner border crossing with Italy.

The EU Commission has expressed concern over the effect this could have on the viability of the passport-free Schengen zone.

Merkel under pressure

German Chancellor Angela Merkel, whose country has accepted more than one million asylum seekers since last summer, was instrumental in pushing the EU-Turkey deal.

Since the deal was put in place, arrivals in Germany have rapidly declined. About 16,000 migrants arrived in April, compared with 120,000 arrivals last December.

But despite the deal's apparent success at stemming the flow of migrants, two-thirds of Germans oppose a fourth term for Merkel.

Only 36 percent of respondents to a poll carried out by Insa for the German magazine Cicero said they wanted Merkel and her Christian Democrats (CDU) party to lead the government following federal elections in 2017.

Merkel has also been under fire for what her critics say is a submissive attitude towards Turkey.

She allowed the prosecution of German comedian Jan Boehmermann following Ankara's request, for insulting the Turkish president Recep Tayyip Erdogan in a satirical poem.

Support for the CDU and its Bavarian sister party, the Christian Social Union (CSU), has fallen to 33 percent.

German magazine Der Spiegel reported that the CSU, which has been critical of Merkel's refugee policy, might run a separate campaign from the CDU in the 2017 election.

Merkel's coalition partner, the Social Democrats (SPD), have also seen a drop in their approval rating to 20 percent.

The party making gains is the anti-immigrant Alternative for Germany (AfD) that moved from a marginal party to having 15 percent support nationally.

Meanwhile, German authorities are investigating claims that 40 Islamist militants were among the asylum seekers who have travelled to the country.

Police said they had received 369 reports of possible extremism, and found that roughly one-tenth of the cases needed to be investigated.

Hans-Georg Maassen, the head of Germany's domestic intelligence agency, had warned that the Islamic State militant group was attempting to “send a political signal” by using the refugee route to stoke fears in Europe.