Turkish President Recep Tayyip Erdogan on Thursday warned the European Union that Ankara would not implement a key deal on reducing the flow of migrants if Brussels fails to fulfil its side of the bargain.

Erdogan's typically combative comments indicated that Ankara would not sit still if the EU falls short on a number of promises in the deal, including visa-free travel to Europe for Turks by this summer.

Meanwhile, the Vatican confirmed that the pope would next week make a brief, unprecedented trip to the Greek island of Lesbos where thousands of migrants are facing potential deportation to Turkey under the deal.

"There are precise conditions. If the European Union does not take the necessary steps, then Turkey will not implement the agreement," Erdogan said in a speech at his presidential palace in Ankara.

The March 18 accord sets out measures for reducing Europe's worst migration crisis since World War II, including stepped-up checks by Turkey and the shipping back to Turkish territory of migrants who land on the Greek islands.

In return, Turkey is slated to receive benefits including visa-free travel for its citizens to Europe, promised "at the latest" by June 2016.

Turkey is also to receive a total of six billion euros in financial aid up to the end of 2018 for the 2.7 million Syrian refugees it is hosting.


'Need more than thanks'

Marc Pierini, visiting scholar at Carnegie Europe, described the visa-free regime as one of the "biggest benefits for Turkey" in the migrant deal.

He told AFP that Turkey still has to fulfil 72 conditions on its side to gain visa-free travel to Europe's passport-free Schengen zone and that the move would also have to be approved by EU interior ministers.

Turkey's long-stalled accession process to join the EU is also supposed to be re-energised under the deal. But Pierini said there were many conditions still to be fulfilled here.

"The worst reading of the EU-Turkey deal would be to imagine that Turkey is about to get a 'discount' on EU membership conditions just because of the refugees," he said.

Erdogan argued Turkey deserved something in return for its commitment to Syrian refugees, on whom it has spent some $10 billion since the Syrian conflict began in 2011.

"Some three million people are being fed on our budget," the president said.

"There have been promises but nothing has come for the moment," he added.


Pope to visit Lesbos

The first transfer of more 200 migrants from the Greek islands of Lesbos and Chios to Turkey took place on Monday.

Officials said Greece was preparing to send around 50 more on Friday unless they applied for asylum at the last minute.

German Chancellor Angela Merkel, whose country took in 1.1 million asylum seekers last year, delivered a message of optimism Thursday regarding the migrant crisis.

"I am very happy today, however I know that we have not yet completed all the tasks before us," she told a press conference during a meeting with French counterpart Francois Hollande in eastern France.

Under the pact with the EU, Turkey has agreed to take back migrants who arrived in Greece in illegal crossings of the Aegean Sea after March 20.

For every Syrian refugee among those sent back to Turkey, one Syrian is supposed to be resettled in Europe.

The deal has caused huge controversy, with rights groups including Amnesty International claiming Turkey could not be considered a "safe country" for the return of refugees.

A Spanish far-left party filed a court complaint Thursday against acting Prime Minister Mariano Rajoy for "crimes against humanity" over his support of the accord.

The Izquierda Unida (IU) party accused Rajoy and other European leaders of "having agreed with Turkish authorities to forcibly deport and transfer an unspecified number of people from EU territory."

According to lawyers for IU, this represents a crime against humanity under the Spanish penal code.

Pope Francis will visit Lesbos on April 16 to show his support for the migrants, the Vatican said.

The Greek government Thursday started to move some migrants from the Moria camp on Lesbos, where 3,000 are crammed, to the open camp of Kara Tepe, also on the island, an official said, insisting it was not linked to the pope's visit.