At least 16 people, including six children, drowned on Saturday after a small boat capsized in the Aegean Sea, Greek coast guard officials said. The boat was ferrying migrants from Turkey to Greece when it went down overnight off the island of Agathonissi. The Greek authorities were alerted to the latest episode shortly after 8 a.m. when three survivors — two women and a man — swam to the island of Agathonissi and said that a boat carrying 21 people had sunk, the coast guard spokeswoman said. “We don’t know where they came from,” she said, adding that no further details were immediately available about the victims’ nationalities. A large search-and-rescue operation was underway, involving five coast guard vessels, a boat belonging to the European Union’s border monitoring agency Frontex, a Greek air force helicopter, a Greek navy ship and four fishing boats, the spokeswoman said. Another two migrants died on Saturday near the land border with Turkey when the truck they were traveling in overturned trying to avoid a police check, the authorities said. The tragedy in the Aegean — the first such mass casualty in several months — came almost exactly two years after Turkey and the European Union signed a deal to curb the flow of migrants trying to reach Europe via the Aegean Sea. Frontex said last month that in 2017, the European Union saw the lowest number of detected illegal border crossings since the migrant crisis began four years ago. The number dwindled to 204,700 from 1.8 million in 2015, it said. The drop was especially observed on the eastern Mediterranean migratory route between Turkey and Greece, and the central route between Libya to Italy. The EU-turkey deal has been criticized by humanitarian groups deterring people from coming who under international law must be granted asylum, such as those fleeing war-torn countries like Syria. Under the agreement, all new irregular migrants crossing from Turkey into Greek islands must be returned to Turkey. These include both refugees fleeing conflict and persecution as well as economic migrants. In addition to providing billions in funds in return, the EU agreed on other concessions to Turkey such as to accelerate plans to bring in visa-free travel for its nationals and boost negotiations for its membership of the bloc. But these have stalled due to Brussels charges that Ankara has committed massive human rights violations in the wake of a failed coup in July 2016. Greece is also detaining thousands of migrants in overcrowded squalid camps.