By Takis Tsafos and Helen Maguire

Athens (dpa) - 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 to allow a further 3,200 to transit through the country, sparking condemnation from bloc leaders pushing for a unified European response.

"Trapping asylum seekers in Greece is an unconscionable and short-sighted non-solution that is causing suffering and violence," said Eva Cosse, Greece specialist at Human Rights Watch.

"It demonstrates once again the EU's utter failure to respond collectively and compassionately to refugee flows."

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 accumulated 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 it calls for an even distribution of refugees across all member states.

If border restrictions to the north 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.

Indirectly criticizing the border restrictions of Austria and several Western Balkan countries, UNHCR's Adrian Edwards said that, "Europe is on the cusp of a largely self-induced humanitarian crisis."

Meanwhile, the European Commission was due Wednesday to pave the way for emergency aid within the bloc. Emergency support has up until now not been available to its own member states.

"This is a necessary step ... to prevent humanitarian suffering as a result of unprecedented numbers of people arriving in the EU," spokesman Margaritis Schinas said, without giving further details.

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, Schinas added.

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."

EU President Donald Tusk was set to start a tour of the Western Balkan in Austria and Slovenia on Tuesday, with the aim of getting countries along the main migration route to back a common EU refugee policy.