A boy eats an apple next to a map illustrating part of Europe, at a makeshift camp for refugees and migrants at the Greek-Macedonian border near the village of Idomeni, Greece, May 19, 2016. [Photo/Agencies]

ISTANBUL - The UN refugee agency (UNHCR) said on Monday border closures in Europe to stop migrants were inhumane and government efforts to stem the flow had averted the crisis only temporarily.

Border closures across the Balkans and a deal between Turkey and the European Union have sharply reduced the number of people crossing into Europe this year, after a million made the often perilous journey in 2015, most fleeing war and poverty in the Middle East and North Africa.

"There are a lot of people patting themselves on the shoulder and saying the deal worked, the people have stopped coming: but there's more to it than that," Melissa Fleming, spokeswoman for the UNHCR, said on the sidelines of the world's first humanitarian summit.

"It has pushed the problem backwards and the problem is not yet solved."

She added: "The sudden (border) closure and the action by unilateral states was inhumane vis-a-vis many vulnerable people."

Under the deal between Europe and Turkey, Ankara has agreed to take back illegal migrants from Europe in return for aid, accelerated EU accession talks and visa-free travel to the bloc.

EU Commissioner for Humanitarian Aid and Crisis Management Christos Stylianides acknowledged there was "room to improve" the bloc's policy on migration. "We have time in order to find this common policy," he said.

Turkey has taken in nearly 3 million refugees since the start of the Syrian civil war and spent nearly $10 billion. Some aid groups say it is not a safe country for refugees.

Last week, a Syrian on the Greek island of Lesbos won an appeal against a decision to forcibly return him to Turkey, successfully arguing that Turkey does not afford refugees the full protection required under the Refugee Convention, rights group Amnesty International said.

An official at Greece's asylum service said the ruling was made in an individual case and was not a decision on Turkey's overall status as a safe third country.

Finalisation of the EU-Turkey deal has been held up by disagreements over Turkey's anti-terrorism law, which Brussels wants brought in line with European standards.

Billed as the first of its kind, the United Nations summit in Istanbul aims to develop a better response to what has been called the worst humanitarian crisis since World War II.