The U.N. refugee agency says it's suspending some operations in Greece because centers known as "hotspots" to handle an influx of migrants "have now become detention centers" under a European Union deal to send back some to Turkey.

UNHCR spokeswoman Melissa Fleming says the refugee agency opposes mandatory detention. She said UNHCR would also halt transport to and from the centers.

She said UNHCR is concerned that Greece doesn't have sufficient structures in place to process migrants properly.

The EU-Turkey deal, in which many people who cross the Aegean will be returned to Turkey, took effect over the weekend.

Separately, UNICEF expressed its frustration over the EU-Turkey plan for making no mention of children, who make up 40 percent of people stranded in Greece.