BRUSSELS (Reuters) - European Commission President Jean-Claude Juncker delivered a personal warning to Turkish President Tayyip Erdogan on Tuesday that he would be held responsible if Ankara fails to meet EU criteria for granting Turks visa-free travel.

An EU offer to liberalize its visa system was part of a deal in March under which Turkey has agreed to all but halt the flow of Syrian refugees and other migrants into the EU via Greece.

But, Juncker said in a speech at Bruges that visas would not be eased if Turkey fails to meet criteria, including amendments to its anti-terrorism law. "Everything the Turkish authorities are doing today leads me to believe that in the end Turkey does not want to be ready to respect European standards," he said.

And if visas were not eased, he said, Erdogan would have to explain to Turks why that was so. "We need Turkey ... but we cannot give up on our main principles," Juncker concluded.

(Reporting by Alastair Macdonald; editing by Philip Blenkinsop)