UN Secretary General Ban Ki-moon on Saturday praised Greece for its handling of the refugee crisis, as well as the solidarity and generosity that it has shown towards migrants.
Ban met Greek Prime Minister Alexis Tsipras in Athens on Saturday, and will later travel to the eastern Aegean island of Lesbos to visit a reception centre for migrants.
Tsipras asked Ban for the UN to use its influence to ensure that the migration deal between the European Union and Turkey remained intact, according to Greek media reports.
The deal was struck in March after more than 1 million migrants and asylum seekers reached European shores last year, many by taking boats from Turkey to Greece. Turkey agreed to take such people back, in return for aid and benefits.
Migrant crossings from Turkey to Greece have dropped from a daily average of 1,740 to just 47 since May, the European Commission said last week.
However, the future of the deal hinges on the EU's ability to grant Turkish citizens visa-free access to the bloc. The benefit is a key condition for Turkey, but Ankara is refusing to meet one of the EU's benchmarks, requiring a narrower definition of terrorism.
Greece came under fire for its handling of the migration influx last year, as the cash-strapped country was accused of failing to control arrivals or prevent people from continuing unhindered towards wealthier northern European states.
Since then, Athens has improved its asylum system and reception conditions, the commission said, but added that Greece must still make "significant" progress to meet EU standards.