The Balkan trail from Greece to northern Europe used by floods of asylum seekers has been blocked after a string of nations slammed shut their borders, hiking pressure on the European Union and Turkey to nail down their "game-changing" deal.

Key points: Balkan states close borders

Balkan states close borders Greece struggles with trapped asylum seekers

Greece struggles with trapped asylum seekers Migrant flow from Turkey continues

Migrant flow from Turkey continues Pressure mounts on EU-Turkey deal

Slovenia and Croatia, two of the countries along the well-trodden route, said no asylum seekers wishing to transit towards other countries would be allowed to enter after midnight (local time) on Tuesday.

This was followed by Macedonia, which then announced yesterday that its border had closed "completely", while Serbia indicated it would follow suit.

Slovenian Prime Minister Miro Cerar said the move meant that "the [Balkan] route for illegal migrations no longer exists".

Croatia's Interior Minister Vlaho Orepic called it a "new phase in resolving the migrant crisis".

Meanwhile, Hungary, which borders the Balkan states to the north, deployed more police and army troops to patrol its southern borders.

The measures follow Austria's decision in February to cap the number of asylum seekers passing through its territory, and Foreign Minister Sebastian Kurz welcomed the news.

"This is putting into effect what is correct, and that is the end of the 'waving through', which attracted so many migrants last year and was the wrong approach," Mr Kurz said.

"As Europe, we must help Greece, but we have to make sure that arriving in [the Greek island of] Lesbos doesn't mean a ticket to Germany," he said.

In Greece, however, the tightening of border restrictions in recent weeks sparked by Austria's move has created a bottleneck at the border with Macedonia where more than 13,000 people were stranded.

There was no official reaction from Athens to Slovenia and Croatia's moves, but a Greek government source said it now considered borders through the Balkans as "de facto closed".

The authorities were trying "to convince the refugees that are stuck to go temporarily to welcome centres throughout Greece," the source said.

The Balkan trail, from Greece to northern Europe. ( ABC News/Wikimedia Commons )

EU, Turkey deal fails to deter asylum seeker flow

Meanwhile, the proposed deal to return migrants and refugees denied asylum in Europe back to Turkey has failed to deter the flow across the Aegean into the EU.

Turkey's coastguard reported that it intercepted dozens of mostly Syrian asylum seekers in coves along the Aegean coast yesterday.

A group of 42 people, more than a dozen of them children, sat inside a coastguard compound, some lying under blankets, in the seaside resort of Didim after being detained.

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Scores more waited among boulders by the beach, watched by armed police, as a bus came to take them away.

"We're afraid of staying here and afraid of staying in Syria," said Sameeha Abdullah, one of the group near the beach, as babies cried around her.

"We're fleeing to the country that will take us. We want safety, someone to care for us."

Abdullah said sanctuary remained elusive, despite her escape from the Syrian civil war.

"We're putting ourselves in the most dangerous situations so we can leave," she said.

But European Commission chief Jean-Claude Juncker called the prospective deal a "real game-changer" and insisted it was "legally feasible".

It has sparked concern from UN refugee chief Filippo Grandi and others.

"As a first reaction, I'm deeply concerned about any arrangement that would involve the blanket return of anyone from one country to another without spelling out the refugee protection safeguards under international law," Mr Grandi told the European Parliament.

Rights group Amnesty International said the proposal was full of "moral and legal flaws" and along with Human Rights Watch challenged the idea that Turkey was a "safe country" to which migrants could return.

Thousands of asylum seekers continue to arrive on Greek islands. ( ABC News: Claire Stewart )

AFP/Reuters