ATHENS (Reuters) - Greece must speed up winter preparations at refugee camps on islands in the Aegean Sea where there has been a sharp rise in arrivals, the United Nations refugee agency said on Friday.

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Nearly 5,000 refugees, mostly Syrian or Iraqi families, crossed from Turkey in September - a quarter of all arrivals this year, UNHCR data shows.

While that is a fraction of the nearly 1 million who arrived in 2015 - due to a European Union deal with Turkey to block that route - four of the five island camps are hosting two or three times as many people as they were designed for.

“UNHCR urges action on the islands to ease overcrowding, improve shelter, and stock and distribute appropriate and sufficient aid items,” said Philippe Leclerc, UNHCR representative in Greece.

In the Moria camp on the island of Lesbos, one of the main entry points, more than 1,500 people are in makeshift shelters or tents without insulation, flooring or heating, UNHCR said. They include pregnant women, people with disabilities, and very young children.

On nearby Samos, about 400 people are living in “very difficult” conditions and another 300, including families and lone children, are sleeping in tents in the woods due to a lack of space in the camp, UNHCR said.

More than 3,000 people on Samos are crammed into facilities designed to hold 700.

In January, refugees in Greece suffered sub-zero temperatures when an icy spell gripped parts of the country and scores of summer tents were weighed down by snow.

More than 60,000 refugees and migrants have been trapped in Greece since Balkan countries along the northward overland route to western Europe sealed their borders in March 2016.

UNHCR has been gradually reducing its involvement on the islands since national institutions took over most services in August.