PRAGUE/BRUSSELS (Reuters) - Eastern European leaders are set to offer manpower and other aid to help Macedonia seal its Greek border, sources close to the discussions told Reuters, in a move that could strand migrants in Greece in the coming months.

The leaders of Hungary, Poland, Slovakia and the Czech Republic -- known as the Visegrad Group within the European Union -- could announce the move when they meet their Macedonian counterpart at a summit in Prague on Monday, diplomats said.

As the European Union gave notice to Athens on Friday that its failure to control hundreds of thousands of refugees landing via Turkey over the past year will see a long-term suspension of some passport-free travel in Europe, EU officials said they expected more border tightening by Greece’s Balkan neighbors.

“Some form of heavy control is in the making,” said one.

Visegrad EU states have led criticism of efforts, notably by Germany, to absorb asylum-seekers who have trekked north out of Greece through Macedonia and Serbia, neither of which are in the EU. Outspoken, right-wing Hungarian Prime Minister Viktor Orban urged Macedonia and Bulgaria last month to follow his example and fence off their borders against refugees and other migrants.

And while Orban’s action was condemned by many European leaders and human rights groups, the idea of preventing people entering impoverished Macedonia and instead holding them in EU-member Greece until they can either be offered asylum elsewhere -- or deported -- has gained ground among policy-makers as they try to prevent new chaos when arrivals rise with better weather.

Austrian Foreign Minister Sebastian Kurz told Macedonians in Skopje on Friday that they should be ready to “completely stop” the entry of migrants in the next few months because he expected Vienna to do the same once it reaches a self-declared limit on the number of migrants it will accept over its eastern borders.

DOMINO EFFECT

Concern at a “domino effect” of border closures rippling down the Balkan peninsula to Greece and leaving large numbers of Syrians, Iraqis and others stranded in some of Europe’s poorest countries, has prompted the EU to offer aid and cooperation to those states, all of them candidates to join the bloc.

Its border agency Frontex is operating with Greek forces on the southern side of the Macedonian border but is prevented by its legal mandate from doing so in Macedonia itself -- prompting Brussels to encourage member states to offer bilateral help.

Another EU diplomat expressed hope that elements of the EU’s so far largely ineffective efforts to curb the influx may soon start falling into place, including the new idea to engage the Visegrad four more in common efforts by helping Macedonia: “If this gets them back on board with European efforts on migration, that would be important politically,” the diplomat said.

Some Visegrad states already have small security force presences on the Macedonian side. They may also look at helping Greece’s other northern neighbors, Bulgaria and non-EU Albania.

Sealing the Macedonian border would raise a prospect of many more people being blocked in Greece. Macedonia has already tried to keep out those who have less prospect of refugee status.

EU officials said delaying people in Greece could help kick start an EU scheme to relocate asylum seekers from Greece to other EU states. Since it was set up, most migrants have preferred to bypass that process and head straight for Germany.

The second diplomat said: “Greece would be the place where things would really happen. We would need to help to build more reception facilities, more infrastructure. And we would do that.”