German Chancellor Angela Merkel has renewed her criticism of moves by other European countries to shut borders to migrants, which she said will not create a sustainable solution.
Austria's decision to impose a cap on refugee numbers set off a chain of border closures that shut the Balkan route used by migrants to trek to central Europe.
Mrs Merkel said: "This unilateral decision by Austria and subsequently by the Balkan countries on one hand brings us fewer refugees, but on the other hand puts Greece in a very difficult situation. And this situation is not durable and sustainable."
She added: "The problem is not solved by one (country) making a decision; it must be a decision that is right for all 28."
Her criticism came as the European Union said 23 of its member countries must start taking 6,000 refugees each month from Greece and Italy to ease Europe's migrant burden.
EU nations committed in September to relocate 160,000 refugees from Greece and Italy over two years to ease their burden, but seven months on fewer than 900 people have actually been shared.
The scheme is key to managing the migrant influx, and the EU's top migration official, Dmitris Avramopoulos, said that "if relocation does not work then the whole system will collapse".
Meanwhile, authorities in Macedonia said 472 people, mostly from Syria, remain stranded in no-man's land on the border with Serbia for a third day as a dispute between the two countries continued over which side should provide shelter for them.
The United Nations refugee agency said more than half of those stranded are children and described the conditions they are living under as "alarming", with migrants sleeping in small tents in a muddy field between the two countries.
Austria and Balkan countries on the route from Greece to northern Europe began imposing border restrictions for migrants last month and halted crossings completely this week, following a meeting of leaders from the European Union and Turkey.
The UN human rights chief has denounced the growing "race to repel" migrants and refugees by some European governments, and said he plans to raise his concerns in Brussels before an EU summit next week.
In his annual report to the Human Rights Council, Zeid Raad al-Hussein said a draft EU-Turkey agreement on handling the migration crisis announced this week raises serious concerns, including "the potential for collective and arbitrary expulsions" from the bloc.