President of the European Council Donald Tusk has confirmed all EU leaders have reached an agreement with Turkey's prime minister to curb the migration crisis.
The new arrangements could come into force as soon as Sunday after a revised deal was reached between Turkish PM Ahmet Davutoglu and European Council president Donald Tusk in Brussels.
Greece's capacity to receive people and assess asylum claims must be strengthened for the deal between the EU and Turkey to work, the United Nations has said.
It also said Turkey must give a fair and timely review to those needing protection.
In a statement, the UN High Commissioner for Refugees (UNHCR) said applicants in Greece should have "the right to appeal before any readmission to Turkey".
The agreement involves closing the EU border on March 20 and sending back migrants who arrive in Greece after that date to Turkey.
Cameron asks EU leaders to back deployment of more international patrol ships close to Libyan coast in bid to deter migrant smugglers.
David Cameron will tell his European colleagues that the migration route from Libya to southern Italy is about to begin once more.