Greece must take "urgent" steps to improve "miserable" conditions in its overcrowded island migrant camps, the UN refugee chief said Wednesday.

"These living conditions need urgent improvement," United Nations High Commissioner for Refugees Filippo Grandi said after visiting camps on Lesbos.

"Whatever the situation of these people, we cannot accept that they live in such miserable conditions, exposed also to violence and exploitation," Grandi said.

There are over 39,000 asylum-seekers in Greek island migrant camps, the government says, with hundreds more arriving daily
There are over 39,000 asylum-seekers in Greek island migrant camps, the government says, with hundreds more arriving daily AFP / ARIS MESSINIS

According to government figures, there are over 39,000 asylum-seekers on the islands, with hundreds more arriving daily, capitalising on mild weather.

The International Organisation for Migration says there are an additional 22,000 people in camps on the mainland, which are nearly full or already past capacity.

Greece's conservative government last week announced a plan to expand and fortify migrant camps on Lesbos, Samos, Chios, Kos and Leros.

United Nations High Commissioner for Refugees Filippo Grandi called for an urgent improvement in conditions at Greece's island camps
United Nations High Commissioner for Refugees Filippo Grandi called for an urgent improvement in conditions at Greece's island camps AFP / ARIS MESSINIS

They also said they would work to make the borders 'air-tight' against a feared new surge of asylum-seekers.

Prime Minister Kyriakos Mitsotakis has also pledged to quickly resettle over 4,000 unaccompanied children after failing to persuade EU states to accept around 3,000 of them.

Grandi, who will see Mitsotakis later on Wednesday and hold a press conference on Thursday, said he "fully" supports plans to bring more children to the mainland.

"I am very worried and I know the PM is very worried about children...we need to do our maximum, all of us collectively, to help the children," he said.

"Europe has to do more for Greece, this is part of the European Union," he added.

There is already opposition from authorities on Lesbos and Samos to the planned new camps that are to hold over 5,000 asylum-seekers each.

Protests have taken place in several towns in northern Greece in recent weeks to block attempted relocations of asylum-seekers.