Four Balkan countries announced daily caps on migrant arrivals on February 26, prompting criticism from the head of the United Nations.
Slovenia, Croatia, Serbia, and Macedonia said they would each restrict the number of migrants allowed to enter their territory to 580 per day.
Slovenia said the new daily limit on migrant numbers was in line with a deal reached on February 18 between police chiefs of Austria, Croatia, Serbia, and Macedonia.
The clampdown follows moves by Austria last week to impose a daily cap of 80 on asylum applications and allow only 3,200 migrants to transit the country each day.
UN Secretary General Ban Ki-moon voiced concerned about the wave of border restrictions in the Balkans, and said they run counter to the international refugee convention.
The daily caps "are not in line" with the 1951 convention "because individual determination of refugee status and assessment of individual protection needs are not...possible" under such a regime, said Ban's spokesman Stephane Dujarric.
Ban is "fully aware of the pressures felt by many European countries" faced with tens of thousands of migrants to deal with each day, Dujarric said. But he believes those countries nevertheless should "keep their borders open, and act in a spirit of responsibility, sharing, and solidarity, including through expanding legal pathways to access asylum."
The caps on migrant arrivals have fuelled a bitter dispute within the European Union, particularly between Austria, which started the current round of restrictions, and Greece, which has been left holding the bag with tens of thousands of migrants stranded on its border with Macedonia.
Athens accuses Vienna of unleashing a domino effect of border restrictions along the so-called "Balkan route" that migrants have been taking to get to northern European countries like Germany and Sweden where they hope to win asylum.
Austria and the Balkan countries, in turn, accuse Greece of failing to properly police its borders, which also are the EU's borders with non-EU countries like Turkey and Syria where most of the migrants originate.
Close to 120,000 migrants have already arrived in Europe so far this year, adding to the one million who made the dangerous journey across the Aegean Sea from Turkey in 2015, according to UN estimates.
The massive influx has boosted nationalist and anti-immigrant populist parties across Europe, dividing the EU's 28 member states, and throwing the future of the EU's passport-free Schengen zone into doubt.
In an effort to bridge the growing divide, EU president Donald Tusk's office announced he will visit Austria, Greece, and the Balkan states next week.
Athens, with backing from other southern EU countries, says it is unable to stop migrants crossing its sea borders without endangering their lives, so the flow of migrants continues. An estimated 2,000 people — more than half from Syria and Iraq — are arriving daily from Turkey on small boats.
But the number of people crossing into neighboring Macedonia has dropped dramatically to just 150 on February 25 and none on February 26, Greek police said, leading to a growing mass of migrants piled up on the border with Macedonia.
Thousands have been sleeping outside in city parks and along the country's highways since Greece's existing migrant shelters are filled to capacity.
Greece said on February 26 that it will try to house migrants on the islands where they land by boat from neighboring Turkey until the border situation is resolved.
Athens ordered ferry companies and authorities on islands near Turkey to restrict the number of migrants allowed to travel to the mainland by ferry.
Deputy Education Minister Sia Anagnostoipoulou told state-run ERT television that Greece could turn into a "giant refugee camp" because of the restrictions to the north.
"What are we supposed to do: Let people drown in the Aegean Sea?" she said. "Instead of making a plan. Europe is burying its head in the sand... Europe is unraveling."
With reporting by AFP, AP, and dpa