Brussels (AFP) - Fears are mounting in the European Union that its months-old pact with Turkey to curb migrant flows could collapse as a rift deepens over Ankara's crackdown following a failed coup.

Turkey angrily rejects EU criticism that its post-coup purges might violate rights norms Ankara must meet under the agreement in return for visa-free travel for Turks and accelerated negotiations for bloc membership.

"The risk is big. The success so far of the pact is fragile," European Commission chief Jean-Claude Juncker told Austrian daily Kurier last weekend.

Turkish President Recep Tayyip Erdogan has "already hinted several times that he wants to scrap it," Juncker said.

If that happens, he added, "then we can expect migrants to start coming to Europe again."

If the pact collapses, cash-strapped EU member Greece would be the first to feel the heat as it was the main gateway for some one million asylum seekers entering Europe last year.

"Clearly we are concerned," Greece's immigration minister Yannis Mouzalas said.

"But for now the number of people arriving on the Greek islands does not indicate that the deal is not being respected," he said.

Since the migrant deal was agreed in March to "break the business model" of smugglers, the numbers of Syrians, Iraqis and others fleeing war and turmoil via Turkey to Greece has slowed to a trickle.

Under the plan, Turkey stops refugees' dangerous sea journeys and takes back illegal migrants from Greece. For each Syrian it accepts back, it can send one to the EU in a more orderly redistribution programme.

In return, Ankara was promised not only visa-free travel for its citizens and accelerated negotiations for membership but also 3.0 billion euros ($3.4 billion) to help the three million migrants on its territory.

But Ankara accuses Brussels of failing to stick to its side of the bargain.

Erdogan on Tuesday lashed out at the European Union for dragging its feet in releasing the promised funds and on visa-free travel.

Foreign Minister Mevlut Cavusoglu said in an interview with the German daily Frankfurter Allgemeine Zeitung that Ankara could withdraw from the accord if Europe failed to allow visa-free travel for Turks by October.

The EU vows not to compromise on its human rights benchmarks, including requiring Ankara to amend its anti-terrorism laws to meet European standards, particularly by narrowing their scope.

However, Turkey rejects the demands and has arrested dozens of journalists under anti-terror laws since the July 15 coup attempt.

More than 60,000 people in military, judiciary, civil service and education sector have been dismissed, detained or are currently under investigation.

-'Serious foreign policy mistake' -

Erdogan has also alarmed European leaders by suggesting Turkey might reinstate the death penalty to punish coup plotters.

"Let me be very clear... no country can become an EU state if it introduces the death penalty," EU foreign affairs chief Federica Mogherini said bluntly in response.

Turkey abolished the death penalty in 2004 as part of its efforts to join the EU, which makes its removal a non-negotiable pre-condition for membership.

EU enlargement commissioner Johannes Hahn warned Turkey last week that the bloc would freeze the country's accession talks if its post-coup crackdown violates EU rule of law norms.

Austrian Chancellor Christian Kern then called Thursday for the EU to discuss ending accession talks, saying "Turkey's democratic standards are far from sufficient to justify" membership.

Turkey's EU Affairs Minister Omer Celik denounced Kern's comments as "extremely disturbing."

Seeking to calm things down, Juncker swiftly rejected Kern's call and warned that ending the negotiations would be a "serious foreign policy mistake."

He did not elaborate what the consequences would be but the EU not only needs Turkey's cooperation on migrants, it is also a bulwark against the Islamic State group in Syria and Iraq.