Another 100,000 spaces in refugee reception centres will be created under a deal agreed by European leaders at an emergency summit in Brussels.
The heads of 11 EU states and three non-EU countries met to discuss how to handle growing numbers of migrants.
More than 9,000 migrants arrived in Greece every day last week, the highest rate so far this year.
Under the deal, Greece will open reception centres with enough room for 30,000 migrants by the end of the year.
The UN's refugee body, the UNHCR, will provide another 20,000 spaces in the same time.
It will also add reception centres with another 50,000 spaces in Balkan countries, which are the most popular routes for migrants looking to travel north to Germany and Scandinavia.
Also as part of the deal, leaders agreed to:
- within a week, send 400 police officers to Slovenia, which has struggled with arrival numbers
- "discourage" the movement of migrants to neighbouring countries' borders "without informing neighbouring countries"
- appoint contact officers who can submit information on migrant numbers to other countries and authorities
"This is one of the greatest litmus tests that Europe has ever faced," said German Chancellor Angela Merkel.
Smaller countries along the Balkan route say their resources are stretched by the number of people arriving.
Bottlenecks have also been exacerbated in part by Hungary closing its borders with Serbia and Croatia, forcing migrants to seek alternative routes north.
Their journeys have been aided by governments who have helped them move to camps or on to the next border.
Before the talks, Croatian Prime Minister Zoran Milanovic dismissed requests to stop moving migrants on.
"That is impossible, whoever wrote this does not understand how things work and must have just woken up from a months-long sleep," he said.
"Waving them through has to be stopped and that is what is going to happen," European Commission President Jean-Claude Juncker said after the summit.
Mr Milanovic and Slovenian President Borut Pahor had said Sunday's talks would be a success only if they agreed to stricter restrictions on migrants travelling from Turkey to Greece.
But no firm new measures on that front were agreed.
Analysis: Lucy Williamson, BBC News, Brussels
There was grumbling, there was pleading, and there were several sharp retorts.
It was sometimes hard to remember that all the leaders who turned up here today professed to want the same thing: an end to the chaos that increasingly marks the migration route through the Western Balkans.
They don't agree on the way to do it.
Even before the meeting began, Croatia's Prime Minister Zoran Milanovic labelled the plan "unrealistic"; drafted by someone "who had just woken up from a months-long sleep". The solution, he said, lay in Turkey and Greece.
It's a sentiment echoed by many of the leaders here today - that without action from Turkey in stemming the number of people crossing its border into Greece, anything else is just tinkering around the edges.
Read more: Europe's rifts laid bare in Brussels
Vast numbers of migrants have made their way across the Mediterranean to Europe in 2015, creating division in the EU over how best to deal with the crisis.
More than 650,000 migrants are estimated to have arrived by sea so far this year, according to the International Organization for Migration (IOM), but exact numbers are unclear.
The conflict in Syria continues to be the biggest driver of the migration.
A note on terminology: The BBC uses the term migrant to refer to all people on the move who have yet to complete the legal process of claiming asylum. This group includes people fleeing war-torn countries such as Syria, who are likely to be granted refugee status, as well as people who are seeking jobs and better lives, who governments are likely to rule are economic migrants.
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