POROS, Greece — The Greek government is detaining migrants incommunicado at a secret extrajudicial location before expelling them to Turkey without due process, one of several hard-line measures taken to seal the borders to Europe that experts say violate international law.

Several migrants said in interviews that they had been captured, stripped of their belongings, beaten and expelled from Greece without being given a chance to claim asylum or speak to a lawyer, in an illegal process known as refoulement. Meanwhile, Turkish officials said that at least three migrants had been shot and killed while trying to enter Greece in the past two weeks.

The Greek approach is the starkest example of European efforts to prevent a reprise of the 2015 migration crisis in which more than 850,000 undocumented people passed relatively easily through Greece to other parts of Europe, roiling the Continent’s politics and fueling the rise of the far right.

If thousands more refugees reach Greece, Greek officials fear being left to care for them for years, with little support from other members in the European Union, exacerbating social tensions and further fraying a strained economy. Tens of thousands of migrants already live in squalor on several Greek islands, and many Greeks feel they have been left to shoulder a burden created by wider European indifference.