Greece sent back a first wave of migrants to Turkey on Monday under an EU deal to ease its migration crisis that has run into heavy criticism from rights groups.

Greece sent back a first wave of migrants to Turkey on Monday under an EU deal to ease its migration crisis that has run into heavy criticism from rights groups.

At first light, a small Turkish ferry, the Lesvos, and a larger catamaran, the Nezli Jale, steamed out of the island of Lesbos carrying 131 migrants, mainly from Pakistan and Bangladesh, said EU border agency Frontex, which is escorting them to Turkey.

Police and riot officers were present, but "the procedure was very calm, everything was orderly," Frontex spokeswoman Ewa Moncure told reporters at Lesbos harbour.

Another Turkish catamaran carried migrants from the neighboring island of Chios. Officials have not yet confirmed how many people are on board.

Red tents have been set up along the town's harbor side to receive the arrivals.

However, Mustafa Toprak, governor of Turkey's Izmir region, said the migrants would only be staying briefly in Dikili and the resort of Cesme -- a second reception point -- before being moved on.

In Monday's first wave, Turkish Interior Minister Efkan Ala has said his country was ready to receive 500 people and that Greek authorities had provided 400 names, although these numbers could change.

Turkey and the EU clinched the agreement in March, with the 28-nation bloc desperate to stem its worst migration crisis since World War II.

Under the terms of the deal, all "irregular migrants" arriving since March 20 face being sent back, although the accord calls for each case to be examined individually.

For every Syrian refugee returned, another Syrian refugee will be resettled from Turkey to the EU, with numbers capped at 72,000.