Turkey has stepped up a crackdown on people smuggling, arresting 1,300 asylum seekers in a single operation just hours after the country promised to curb the flow of refugees to Greece in exchange for financial aid from the EU.

Hundreds of Syrians, Afghans, Iranians and Iraqis and three people smugglers were seized on Monday in the countryside near Ayvacık, a Turkish town north of the Greek island of Lesbos, Reuters and the Associated Press reported. According to the UN, about 425,000 people have arrived in Lesbos in smuggling boats this year, while a further 300,000 have reached other Greek islands from Turkey – leading the EU to criticise its eastern neighbour for not doing enough to police its own border.

The Ayvacık sweep is thought to be the largest single mass arrest of refugees in recent months, and follows an agreement on Sunday that saw the EU pledge to give Turkey €3bn (£2bn) in exchange for increased border patrols.

Turkey says it has detained nearly 80,000 would-be migrants since 2014 as well as over 200 major smugglers. But rarely is an operation as big and organised as Monday’s sting. Recent Guardian reporting highlighted how police turn a blind eye to the smuggling economy in Izmir, where smugglers do business a few metres from two police facilities. On the beaches near the town of Çeşme, where many leave for the Greek island of Chios, the departure points are unpoliced and accessible to all.

But the arrest of more than a thousand people in one day suggests Turkey is increasing efforts to secure its borders in response to the EU deal. Rights groups warn this development will endanger refugee lives, since those who still want to reach Europe will be forced to try riskier methods.

Melanie Ward, associate director of policy and advocacy for International Rescue Committee UK, said the agreement “is deeply concerning because it is primarily designed to obstruct the movement of those seeking refuge in the EU, which runs contrary to the EU’s basic founding principles. This deal will only make it more expensive and dangerous for those determined to continue their journey to Europe”.

Smugglers have also warned that it is impossible to completely curb such a large flow of refugees. Speaking to the Guardian before the crackdown, one smuggler in Izmir said: “It’s the Syrians who determine whether they’ll go or not. The people risking the journey from Damascus, they’re the ones who are making this happen. Anyone who wants to go will go.”

Turkey is home to more Syrian refugees than any other country, with estimates suggesting it houses between 1.8 million and 2.2 million. Most of them do not have the right to work legally – a factor cited by many refugees as a reason for leaving for Europe. One Syrian who plans to leave Turkey in the coming days said that the status quo means many Syrians work illegally to support their families – and end up being exploited by their employers.

“I worked for three months and never got paid once,” said the Syrian, who asked not to be named. “Because of this I want to leave – so that I can live in dignity.”