Turkey must meet all requirements for visa-free travel with the European Union and the criteria will not be watered down, European Commission President Jean-Claude Juncker has said.
Junker's comments came after Turkey's prime minister Ahmet Davutoglu appeared to warn that a refugee swap deal between the EU and Ankara risked collapse unless visa restrictions were lifted on Turkish nationals by June as promised.
"I maintain my belief that, God willing, we will have the visa exemption in June. In the absence of that, then of course no one can expect Turkey to adhere to its commitments," Davutoglu said.
"Turkey must fulfill all remaining conditions so that the Commission can adopt its proposal in the coming months," Juncker said in response on Tuesday. "The criteria will not be watered down."
OPINION: The dark side of the EU-Turkey refugee deal
Last month, Turkey and the EU reached a deal aimed at stopping the flow of people into Europe.
Under the deal, Ankara agreed to take back all refugees and so-called economic migrants who enter Greece illegally, in return for the EU taking the same amount of Syrian refugees directly from Turkey.
As a part of the deal Turkey wanted to negotiate EU accession, receive $6.8bn of EU funds to house Syrian refugees, and have short-term visas waived for its citizens before the end of June.
But, Turkey has to meet some 72 technical requirements before the short-stay visa restrictions are removed in the passport-free Schengen zone. As of last month, it had only met half of them.
More than 1.1 million people landed on EU shores over the past year, mostly via Greece from Turkey. Daily arrivals have fallen sharply since the deal between Ankara and Brussels.