This is part of the EU's controversial deal with Turkey. The agreement, aimed at stemming the flow of migrants into the EU, has been widely criticized by rights groups.
Turkey faces criticism for border policies allowing jihadists to flow back and forth from Syria. But it deported one of the Brussels bombers in 2015, and says Europe failed to take warnings seriously.
The Greek coast guard reported a sharp drop in the number of migrants crossing between Turkey and Greek Islands on Thursday. Officials believe that bad weather and new restrictions both played a part.
Turkey's President Recep Tayyip Erdogan says Belgium was warned about the terrorist ties of one of the brothers suspected in Tuesday's suicide attack in Brussels.
While the tragedy in Brussels is the focus of headlines around the world, there have been a number of other strikes in the past 10 days in places such as Nigeria and Yemen.
A Turkish columnist once hailed the country's leader as a champion of democracy. Now he joins many Turks in gloom and disappointment over the country's course.
Efkan Ala said Sunday that the attacker who struck Istanbul on Saturday was a Turkish-born member of ISIS. Four victims died in the blast: three Israelis or Israeli-Americans and one Iranian.
The migrant crisis is the latest example of the fraught, tangled relationship. Europe wants Turkey as a buffer that blocks spillover from Middle East chaos. Turkey wants access to Europe's markets.