Chaotic scenes at Greece's border with Macedonia, where thousands of migrants are stranded, are a "direct result" of border closures across Europe and Austria's cap on arrivals, Human Rights Watch said on Tuesday.
The statement came one day after angry crowds stormed a metal fence and police fired tear gas at protesters, in images that crystallized increasing tensions at transit points for migrants seeking a better life in Europe.
"The chaos and violence unfolding on the Greece-Macedonia border are a direct result of discriminatory border closures and Austria's unilateral cap on asylum seekers," the statement said.
Migration restrictions along the so-called Balkan route, the main corridor for refugees and migrants headed further north to richer EU countries, have led to bottlenecks at the border between Greece and neighbouring Macedonia.
Austria late last month decided to cap the number of asylum seekers it will take at 80 per day and allow a further 3,200 to transit through the country, sparking condemnation from bloc leaders pushing for a unified European response.
Beginning his tour of the Balkan migration route in Vienna, EU President Donald Tusk says Europe must return to its open-border policy.
"Today we are opening a new chapter of our struggle with the migration crisis, called 'Back to Schengen'," he said, referring to controls set up in Europe's border-free Schengen area.
Tusk's visit to Austria, Slovenia, Croatia and Macedonia is aimed at getting these countries to reconsider their tough new border restrictions. He is also set to visit Greece on the trip.
Tusk said as he called for support for Greece, which has faced growing pressure from stranded migrants who are no longer able to travel onwards on the Balkan route.
Around 100 migrants gathered to protest the closure of the passage from Greece to Macedonia on Tuesday, but state-run broadcaster ERT reported that further disturbances have yet to meet the scale of Monday's riots.
At the Greek village of Idomeni, where Monday's riots occurred, about 8,000 refugees have already gathered and Greek military forces are rushing to build seven new camps with a maximum capacity of 20,000 people.
Prime Minister Alexis Tsipras said in an interview Tuesday morning with Greek TV station Star that he won't agree to any migration policy with the European Union - including an agreement at the EU-Turkey summit on March 7 in Brussels - unless the burden of incoming refugees is distributed across all member states.
If border restrictions remain and the inflow does not abate, then the number of migrants stranded in Greece may grow to 70,000 by the end of March, the Greek minister in charge of migration, Ioannis Mouzalas, said over the weekend.
In Geneva, the UN refugee agency UNHCR warned that Greece and countries along the Balkan migration route were headed for "disaster" unless they return to a common approach of solidarity and shared responsibility.
"Europe is on the cusp of a largely self-induced humanitarian crisis," UNHCR's Adrian Edwards said.
Meanwhile, the European Commission was due Wednesday to pave the way for emergency aid within the bloc. Emergency humanitarian support has up until now not been available to its own member states.
European Commission President Jean-Claude Juncker promised "unwavering support" to help Athens manage the refugee crisis, in a telephone conversation with Premier Tsipras on Monday, spokesman Margaritis Schinas said.
He said that the commission was "very concerned by the images" it saw at the Greek-Macedonian border on Monday, noting that this was "not our idea of managing the crisis."