EU nations must accept up to 200,000 refugees as part of a “common strategy” to replace their “piecemeal” approach to the migrant crisis, the UN says.
Antonio Guterres, head of the UN refugee agency, said the EU must mobilise “full force” for the crisis, calling it a “defining moment”.
EU leaders, split over sharing the refugee burden, are scrambling to agree a response in meetings on Friday.
In Hungary, hundreds of refugees are locked in a stalemate with authorities.
Migrants hoping to reach the Austrian border have refused to disembark from a train surrounded by police in the Hungarian town of Bicske, 40km (25 miles) from Budapest.
Live updates on the crisis
Hungarian authorities want to move the migrants to a nearby refugee camp – but the migrants fear registering there will hamper their plans to seek asylum in Germany and other countries.
In the Hungarian capital, Budapest, hundreds of stranded refugees have vowed to “walk to Vienna” because they have not been allowed to board trains onwards.
Hungary has also shut its main border crossing with Serbia after some 300 migrants escaped from a camp in the town of Roszke, prompting a police search operation.
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