BELGRADE (Reuters) - More than 1,000 migrants, trapped in Serbia after fleeing war and poverty in the Middle East and Central Asia, have occupied a derelict warehouse in the capital Belgrade, where they are preparing to tough out a bitter Balkan winter.

More than 100,000 migrants have passed through Serbia this year, from countries such as Syria, Iraq and Afghanistan, to seek sanctuary in the wealthy countries of northern Europe. But border closures have left many stranded.

"We want to go to Italy, Germany, France, but the border is closed at Hungary and Croatia," said Najib, who said he was from Afghanistan.

Around him, men warmed their hands on camp fires, as smoke drifted up into the high rafters, and others perched on car tyres as they washed at a makeshift sink in a vain attempt to keep away the filth of the tumbledown building.

Last year, hundreds of thousands of refugees swept through the Balkans, prompting countries to throw up near-impenetrable barriers to block them. More than 6,400 refugees are trapped in Serbia, 1,200 of them in this warehouse, aid workers say.

Some who had made it further north said they were beaten by police at the Hungarian border and sent back to Belgrade. Many had Serbian documents telling them they had no grounds to remain - but few have the means to return home.

"Here, look at this place, it's very dirty. I become sick," said Imrar Khan, 17, who left Pakistan three months ago. Dozens of migrants bed down in rows between piles of garbage in the warehouse, near the city's central station.

Authorities blame people traffickers for much of the influx. Since the beginning of the year, they have prevented 5,000 illegal border crossings and charged 356 people with the smuggling of around 2,000 people.

(Reporting by Thomas Escritt; Editing by Robin Pomeroy)