The European Union’s concerns about illegal migration from the Balkans have continued to grow with numbers doubling since last year.
New figures from EU border agency Frontex said the Balkan route made up 45 per cent of illegal crossings this year.
The number of migrants now taking that path is at its highest since the migration crisis of 2015, it said.
It comes as Austria raises the alarm about the number of migrants slipping through the net to reach its landlocked borders.
The increasing traffic is blamed partly on migrants using favourable visa rules to land in Serbia, then continuing to EU borders.
But Frontex said there were also “repeated attempts to cross the border by migrants already present in the region”.
It said there were 14,105 arrivals from the Balkan region last month, twice as many as throughout last year.
People from Syria, Afghanistan, Turkey and Tunisia are the most common nationalities to use that route this year.
Austria has questioned the official figures by saying many migrants are not being registered until they reach its borders.
Six western Balkan countries — Serbia, Montenegro, Kosovo, Bosnia and Herzegovina, Albania and North Macedonia — are in various stages of applying to join the EU.
The bloc has urged them to align their visa policies with Brussels to close loopholes.
EU members Romania and Bulgaria meanwhile had their applications to join the visa-free Schengen zone vetoed by Austria.
It sparked a war of words as Romania said Austria was playing political games and was in turn accused of failing to get a grip on migration.
In the latest salvo, Romania’s Foreign Minister Bogdan Aurescu told his counterpart Alexander Schallenberg that Austria’s claims were wrong.
Accession to the Schengen zone “cannot be artificially connected” with other issues “for which Romania is not responsible”, the country's Foreign Ministry quoted Mr Aurescu as saying.
Elsewhere, Frontex said illegal border crossings across the entire EU were at their highest level since 2016.
It reported a sharp increase along the Eastern Mediterranean route to Greece and Cyprus.
More than 3,400 people arrived at EU borders by that route last month, typically from Syria, Afghanistan, Nigeria or the Democratic Republic of the Congo.
But numbers were down on the eastern border, where a crisis erupted a year ago where Poland meets Belarus.
Only 334 people were detected trying to enter the EU by that route last month, Frontex said.
Most Ukrainian refugees do not count towards the figures because they generally have legal status in the EU.