The Czech and Slovak prime ministers say the European Union needs a 'Plan B' in case Greece can't manage the influx of migrants through its territory.

Bohuslav Sobotka and Robert Fico say to protect the European visa-free Schengen travel zone in that case, the 28-nation bloc should take necessary measures on Greece's borders with Macedonia and Bulgaria.

More than 850,000 people, most fleeing conflict in Syria and Afghanistan, entered Greece by sea in 2015. Some European countries, including Slovakia, have blasted Greece for being unable to secure its border, which forms part of the Schengen area.

After the two leaders met Tuesday, Fico said: "Macedonia and Bulgaria could play a key role in protecting the Schengen border."

The two countries already agreed to deploy police forces in Macedonia.