A spokesman for Greece’s government, Stelios Petsas, said the barrier system would have to be tested for safety.

But rights activists warn that the measure would increase the dangers faced by migrants making the short but perilous journey across the Aegean. Amnesty International’s research director for Europe, Massimo Moratti, condemned the proposal as “an alarming escalation in the Greek government’s ongoing efforts to make it as difficult as possible for asylum-seekers and refugees to arrive on its shores.”

He warned that it could “lead to more danger for those desperately seeking safety.”

The head of Amnesty International’s chapter in Greece, Gavriil Sakellaridis, questioned whether the Greek authorities would respond to an emergency signal issued by a boat stopped at the barrier.

The European Commission has expressed reservations and planned to ask the authorities in Greece, which is a member of the European Union, for details about the proposal. Adalbert Jahnz, a commission spokesman, told reporters in Brussels on Thursday that any Greek sea barriers to deter migrants must not block access for asylum seekers.

“The setting up of barriers is not in and of itself against E.U. law,” he said. “But physical barriers or obstacles of this sort should not be an impediment to seeking asylum which is protected by E.U. law,” he said, conceding, however, that the protection of external borders was primarily the responsibility of member states.

The barrier was proposed amid an uptick in migrants from Turkey. The influx, though far below the thousands of daily arrivals at the peak of the crisis in 2015, has put an increasing strain on already intensely overcrowded reception centers.

According to Greece’s migration minister, Notis Mitarakis, 72,000 migrants entered Greece last year, compared with 42,000 in 2018. The floating barrier will help curb arrivals, Mr. Mitarakis said.