Leaders of four EU countries that once lay behind the Iron Curtain warned on Monday against "new dividing lines" in Europe and urged cooperation with Turkey on the migrant crisis.

The Czech, Hungarian, Polish and Slovak prime ministers meeting in Prague within the framework of the Visegrad Four (V4) group also agreed a joint position on a deal designed to prevent Britain from leaving the EU.

The moves came as European Union chief Donald Tusk warned Monday there was a "real" danger of the 28-nation bloc breaking up over so-called Brexit and called for greater EU coordination to address the risk.

Czech Prime Minister Bohuslav Sobotka is expected to present the V4 position to Tusk in the Czech capital on Tuesday.

"We are aware of the risks and negative consequences of any potentially emerging new dividing lines in Europe," V4 premiers said in a joint statement.

"We are determined to work actively and resolutely to prevent... new dividing lines."

In a separate statement, they also backed Greece in its efforts to protect the southern border of the passport-free Schengen area and urged the EU to move quickly on an action plan with Turkey intended to curb the flow of refugees and migrants from that country.

Greece announced Monday it would open four "hotspot" migrant registration centres this week to help curb an influx that brought more than a million people to Europe last year.

Sobotka said the EU should coordinate all steps with non-member Balkan states and support steps taken by Bulgaria and Macedonia to protect their borders.

Earlier on Monday, Czech state secretary for EU affairs, Tomas Prouza, tweeted that Macedonian President Gjorge Ivanov had complained "direct assistance from the European Commission is zero, therefore help from Visegrad, Austria, Slovenia important."

Hungarian Prime Minister Viktor Orban said his country was ready to provide "human resources and material to countries that are ready to build a defence line south of Hungary."

His Slovak counterpart Robert Fico offered 300 policemen as border guards in case Greece failed to protect its border, making it necessary for Bulgaria and Macedonia to boost their border controls under what he termed a "plan B".

The leaders also called for "common solutions" and tackling "the root causes of the current migratory pressure, including ending of war in Syria" and wa