Greek Prime Minister Alexis Tsipras is due to meet Turkish President Recep Tayyip Erdogan on Monday afternoon at the sidelines of the first World Humanitarian Summit, which is taking place in Istanbul.
The two leaders are due to meet at 6 p.m. and the EU-Turkey refugee agreement is expected to top their agenda of talks.
Earlier, Tsipras is due to meet Ecumenical Patriarch Vartholomaios, the spiritual leader of Orthodox Christians.
The gathering in Istanbul was conceived four years ago by UN Secretary General Ban Ki-moon and aims to revamp humanitarian aid and global responses to modern-day crises.
In preparation, 23,000 people were consulted in over 150 countries, UN officials say.