BUCHAREST (Reuters) - About 500 migrants from the Middle East reached Romania across the Black Sea from Turkey in about a month, coastguards said on Monday, in a sharp increase the European Union says shows smugglers are testing alternative routes into Europe.

Since 2015, the EU has been struggling to bring down the number of refugees and migrants smuggled or trafficked from the Middle East and Africa to its shores.

A 2016 deal with Turkey shut the route to EU state Greece and the bloc is now cracking down on arrivals from Libya to Italy. All in all, Mediterranean arrivals to the bloc fell from nearly 28,000 people in June to below 10,000 in August, according to U.N. data.

But the Romanian coast guard told Reuters on Monday that 475 people reached its shores in about one month in August-September, including from Iraq and Iran. Seven people smugglers from Turkey, Bulgaria, Syria, Iraq and Cyprus had been arrested.

While still low compared to thousands of migrants reaching Greek islands daily at the height of the arrivals in 2015, the figure marks a sharp increase from a total of 500 migrants who completed the same route in two years in 2013-2015, according to Romanian data.

The EU’s border agency Frontex has said it is monitoring the situation and that people smugglers might be looking for alternative entries to the bloc after the Mediterranean crossings were made more difficult.

“While it is too early to talk about the opening of a new migratory route, the recent incidents of intercepted wooden boats with migrants on the Black Sea suggest that smugglers might be looking to revive this route,” said a Frontex spokesman, Krzysztof Borowski.

Frontex said the Black Sea route was last active in 2014 but did not expect it to develop on a large scale due to difficult sailing conditions on the Black Sea, especially as autumn nears.

On Aug. 13, the Romanian coast guard said it had rescued 101 adults and 56 children from Iraq and Iran crowded onto a small fishing boat, in distress amid strong winds.

“They were packed in there ... The boat could (only) safely

accommodate a dozen people,” a coastguard officer said.

In Romania’s neighbor Bulgaria, interior ministry data showed no illegal migrants had been stopped at its sea border in 2017.

Both countries are still outside of the EU’s Schengen zone of free travel and hope to join as soon as possible.

While the bloc’s executive has said it will soon recommend bringing them in, many other EU states are skeptical. How the two will manage any migratory flows would be crucial to determining whether they meet the criteria.

Since joining the EU in 2007, both are also under monitoring by the EU executive arm in Brussels over justice reforms and concerns over corruption.

While Ankara has cracked down on smuggling across the Mediterranean, many refugees and migrants who had made it across before are being stranded on Greek islands in poor conditions.

As the bloc’s relations with Turkey have grown increasingly strained since the 2016 migration deal, Brussels and EU states worry that President Tayyip Erdogan might one day walk away from the accord.

“Erdogan has just lifted a lid to keep pressure on the EU. It’s flexing muscles,” Romanian political analyst Mircea Marian said of the recent increase in arrivals from the Turkish coast across the Black Sea.