A last-minute rush of asylum applications hampered operations Tuesday to return migrants from Greece to Turkey under an EU deal, as the UN watchdog voiced concern for 13 individuals sent back the day before. Yiorgos Kyritsis, the spokesman for the Greek government panel coordinating the migration crisis, said no operations were planned for Tuesday. They will resume "when there is a sufficient number" of migrants, Kyritsis said, as Turkish officials sketched Wednesday as a resumption date. The procedure has been slowed "by an increase in asylum requests" in the last few days by migrants on Chios and Lesbos, the Greek Aegean islands in the front line of the migratory wave, Kyritsis said. Out of around 6,000 migrants who arrived on the islands after a deadline of March 20, more than 2,300 had applied for asylum, he said. The European Union (EU) signed the controversial deal with Turkey in March, desperate to defuse the continent's worst migration crisis since World War II. More than a million people arrived in 2015, many of them from war-ravaged Syria. The accord aims at curbing the main influx which comes from Turkey, especially those smuggled by illegal gangs on the short but perilous crossing over the Aegean. All "irregular migrants" arriving since March 20 face being sent back, although the deal calls for each case to be examined individually. On Monday, a first batch of 202 migrants, most of them Afghans and Pakistanis, were sent back from Chios and Lesbos in what the EU's border agency Frontex said was a "very calm... orderly" operation. But on Tuesday, the UN High Commissioner for Human Rights (UNHCR) raised fears for some of those deported. "We are concerned that 13 people, most of them Afghans, who expressed the wish to request asylum were unable to be registered in time," the UNHCR's representative in Greece, Philippe Leclerc, said. The UNHCR is checking with the Turkish authorities to see if the 13 request asylum protection, Leclerc said. The 13 people, part of a group of 66 sent back from Chios, may have been victims of the "confusion" that reigned in the migrant camp of Vial on Friday after several hundred people bolted following a brawl between rival groups, Leclerc said. Under the controversial EU plan, for every Syrian refugee returned, another Syrian refugee will be resettled from Turkey to the EU, with numbers capped at 72,000. The idea is to encourage Syrians seeking to flee to Europe to stay safely in Turkey, with the prospect of asylum, rather than try to make the dangerous sea crossing in the hands of ruthless smugglers. Under the one-for-one part of the deal, Germany on Monday accepted 32 asylum-seekers from Turkish soil. Critics of the March 18 deal include Amnesty International, which says Turkey is not a safe country for refugees — a charge Ankara rejects.