The surge of migrants across the Balkans on their way to Western Europe is continuing unabated just hours after an EU-sponsored agreement to slow the flow.

European and Balkan leaders agreed on measures to stem the flow of tens of thousands whose flight from war and poverty has overwhelmed authorities and heightened tension among nations along the route.

Croatia's police said more than 13,000 migrants had arrived from Serbia in the past 24 hours.

About 260,000 have passed through Croatia since mid-September when Hungary put up a fence on the border with Serbia, diverting the flow.

Slovenia's police said nearly 10,000 migrants had entered Slovenia from Croatia in the same period, bringing the total number of arrivals in the past 12 days to almost 75,000.

Earlier, in a statement to paper over the deep divisions about how to handle the crisis, the leaders committed to bolster the borders of Greece as it struggles to cope with the wave of refugees from Syria and beyond that cross over through Turkey.

Reception capacities should be boosted in Greece and along the Balkans migration route to shelter 100,000 more people as winter loomed, they said.

They also agreed to expand border operations and make full use of biometric data like fingerprints as they register and screen migrants, before deciding whether to grant them asylum or send them home.

"The immediate imperative is to provide shelter," European Commission president Jean-Claude Juncker said after chairing the often bad-tempered mini-summit of 11 regional leaders in Brussels.

"It cannot be that in the Europe of 2015 people are left to fend for themselves, sleeping in fields."

"This is one of the greatest litmus tests that Europe has ever faced," German chancellor Angela Merkel said after the summit. "Europe has to demonstrate that it is a continent of values and of solidarity.

"We will need to take further steps in order to get through this."

Slovenian prime minister Miro Cerar said his small Alpine nation was being overwhelmed by the refugees and was not receiving enough help from its EU partners.

He put the challenge in simple terms: if no fresh approach is forthcoming "in the next few days and weeks, I do believe that the European Union and Europe as a whole will start to fall apart".

The leaders agreed to rapidly dispatch 400 border guards to Slovenia as a short-term measure.