Greece sent a first wave of migrants back to Turkey under an EU deal on Monday.

A small Turkish ferry and a larger catamaran, left the island of Lesbos carrying 131 migrants, mainly from Pakistan and Bangladesh, according to EU border agency Frontex.

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

Turkish Interior Minister Efkan Ala has said his country was ready to receive 500 people on April 4 and that Greek authorities had provided 400 names, although these numbers could change.

Greek officials have been tight-lipped over how many migrants will cross the Aegean Sea back to Turkey. Under the same deal, for every migrant taken back from Greece, the EU will receive a Syrian refugee from Turkey.

Some 40 Syrian refugees will be sent to Germany from Turkey on April 4, German Ambassador to Ankara Martin Erdmann told Hürriyet Daily News on April 1. “A group of 40 persons will be sent from Istanbul to Hannover on April 4” Erdmann said.

Greek state news agency ANA reported April 3 that some 250 migrants from Bangladesh, Pakistan, Sri Lanka and African nations would be sent back daily between April 4 and April 6.

Yiorgos Kyritsis, spokesman for Greece’s refugee coordination unit, insisted the April 4 operation only “involves people who have not requested asylum.”

The EU signed the deal with Turkey in March, desperate to defuse the continent’s worst migration crisis since World War II, with more than a million people arriving from the Mideast and elsewhere last year.