The discord is raising hopes among a host of Syrians and others whose plans were sidetracked when the deal was cinched in March. The other day, one of them, a Syrian named Mohammad Ibrahim, drank tea in the shade of a palm tree on the Turkish coast and looked longingly at Greece, visible across a narrow stretch of the Aegean Sea. He was waiting to meet up at night with smugglers who would help him across the waters.

Like many Syrians, he said he had realized that one of the main provisions of the agreement — that migrants who risk their lives on the sea are to be sent back to Turkey — was rarely enforced.

“This agreement is only on paper,” said Mr. Ibrahim, 44, who is from the war-torn city of Aleppo. “In practice, they aren’t sending us back to Turkey like they said they would. So now it is time to go.”

Under the deal, the European Union pledged more than $6 billion to improve the lives of the more than three million Syrians living in Turkey. It also agreed to renew negotiations for Turkey to join the bloc, a prospect that appears dimmer than ever given the tensions between the two sides since the coup attempt.

Turkish leaders feel that Europe should have stood in solidarity with Turkey at a difficult time rather than criticize the crackdown on tens of thousands of people the government said were followers of Fethullah Gulen, the reclusive cleric whom the Turks accused of leading the revolt.

Even before the coup, Europe had been criticized for ignoring Turkey’s crackdown on freedom of expression, and the growing authoritarianism of President Recep Tayyip Erdogan, in order to contain the migrant crisis. After the coup and the purges that followed, it found itself in an even tougher position.

“Instead of unequivocally condemning the coup and supporting the elected government’s efforts to bring the putschists to justice, Europe chose to attack Turkey’s leaders for holding the would-be junta accountable for their crimes,” Ibrahim Kalin, Mr. Erdogan’s spokesman, wrote in a recent column titled “Brussels, You’ve Got a Problem.”