Refugee children trying to reach Europe are being preyed on by paedophiles and forced to resort to prostitution in order to pay people smugglers, it has emerged.

The warning was issued by the UN refugee agency, which said it is concerned at “credible testimonies” which it received of sexual violence and abuse against refugee women and children.

Among the worrying cases reported there are instances of children engaging in what is being termed “survival sex” to pay people smugglers to continue their journey when they have run out of money or been robbed.

The agency said so far this year, more than 644,000 refugees and migrants had arrived in Europe by sea - with just over a third women and children.

The crisis shows little sign of slowing so far, with more than 56,000 people – mainly from Syria – reported to have attempted the risky crossing from Turkey to Greece in the past six days alone.

Melissa Fleming, spokeswoman for the UN High Commissioner for Refugees (UNHCR), said: “Refugee and migrant children moving in Europe are at heightened risk of violence and abuse, including sexual violence, especially in overcrowded reception sites, or in many locations where refugees and migrants gather, such as parks, train stations, bus stations and roadsides

“From testimony and reports we have received there have been instances of children engaging in survival sex to pay smugglers to continue their journey, either because they have run out money, or because they have been robbed.”

She added that unaccompanied children could be particularly vulnerable as they lack the protection and care of an adult. They may also be placed in detention in some countries, including with adults, posing great risks to them, she said.

The UNHCR has appealed to all authorities in Europe to take measures to ensure the protection of women and girls, including providing adequate and safe reception facilities.

Meanwhile Serbia and Croatia yesterday agreed to ease the flow of refugees over the border between the countries after thousands of people, including children, were forced to spend the night in near-freezing temperatures along a muddy border passage.

The interior ministers of Serbia and Croatia said they will start shipping refugees by train directly from Serbia to Croatia so they will not have to cross on foot, with them often trekking for miles.

“We have agreed to stop this torture,” said Croatian interior minister Ranko Ostojic. “There will be no more rain and snow, they will go directly from camp to camp.”

Tensions have been building after a shift in the so-called Balkan route through Europe after Hungary erected fences along its borders with Serbia and Croatia.

While refugees still cross from Greece to Macedonia and onto Serbia, they now go on via Croatia and Slovenia instead of Hungary.

Two United Nations experts yesterday warned that force will not stop Europe’s migration crisis or deter people smugglers.

Francois Crepeau, the UN special investigator on migrant rights, and Francisco Carrion, head of the UN Committee on the Protection of the Rights of Migrant Workers, criticised a resolution by the world body’s Security Council allowing the European Union to inspect and use force against boats in the high seas suspected of being used to smuggle migrants from Libya.

Crepeau said Europe should do for Syrian, Eritrean and possibly Afghan refugees what the US and other countries did nearly 40 years ago for refugees from Vietnam and Indochina and implement massive resettlement programmes from transit countries over many years.

“It’s totally manageable,” he said. “We’ve done it in the past. We can do it again. We’re richer, more populous than we were 40 years ago.”