By Renee Maltezou and Lefteris Papadimas

ATHENS (Reuters) - Greece plans to reduce overcrowding at migrant camps on its islands, a government official said on Wednesday, to ease growing tensions and improve conditions that campaigners have called "disgraceful".

More than 10,700 migrants are now sheltered on five Greek islands which have capacity for 7,450, according to government data. Charity Save the Children said they were living in "dirty, unsafe conditions ... with no end in sight."

Greece now plans to build several new facilities with a capacity of about 1,000 people each, to transfer migrants from the islands and other strained or temporary sites.

A government official said the slow processing of requests for asylum - without which migrants face being returned to Turkey - was adding to building frustration at the camps.

"We are facing a lot of problems on the islands. People feel trapped and disillusion is growing. They came very close to materializing their dream of reaching Europe but it didn't happen," said the official, speaking on condition of anonymity.

"We are moving to a de-congestion process while speeding up the processing of asylum requests," the official said. "The new facilities will be better, more permanent, smaller dwellings."

Greek authorities said late on Wednesday that they had returned by plane to Turkey eight more men from Syria bringing the total number, under the accord agreed with EU in March, to 476.

Including those on the islands, there are 57,679 migrants stranded in Greece, up from about 42,000 in March. Its shelters have capacity for 61,409 people.

Flows have slowed since the deal between the European Union and Turkey came into force in April. There has been a small increase in the past few weeks, but nothing on the scale of the hundreds or even thousands arriving daily last year.

Save the Children said 3,800 of those on the islands were children. The charity called on the European Union to give more funding to help improve conditions for migrants in Greece.

A representative for Arsis, a non-governmental organization providing social support to young people, said there were only 728 beds available for 3,168 registered unaccompanied minors in Greece.

"We are constantly looking for space to provide shelter to unaccompanied minors," said Arsis' Katerina Poutou. "Our needs are urgent."

(Editing by Robin Pomeroy and Toby Chopra)