Some refugee children are being forced into "survival sex" to pay people smugglers, the United Nations' refugee agency has claimed.

The UNHCR says it has received "credible testimonies" of sexual violence and abuse against women and children on the move in Europe, and is urging authorities to take action.

"Refugee and migrant children moving in Europe are at heightened risk of violence and abuse, including sexual violence, especially in overcrowded reception sites," the agency's spokeswoman Melissa Fleming said.

"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 of money, or because they have been robbed."

So far this year, more than 644,000 refugees and migrants have arrived in Europe by sea. A third of those are women and children.

According to the International Organization for Migration, Greece last week received its largest weekly influx of asylum seekers so far this year, with the arrival of 48,000 people.

The situation has prompted several countries to close their borders, to try to force asylum seekers to settle elsewhere.

Germany announced on Friday it was fast-tracking measures aimed at quickly dealing with asylum claims from people in southeastern Europe, so that authorities could instead focus on those fleeing war-torn countries such as Syria, Iraq and Afghanistan.

The new measures will speed up the deportation process of those who have had their asylum claims rejected.

Germany expects to accept more than 800,000 refugees this year, but public support for the plan is falling.

Meanwhile, Turkey says it is preparing for thousands more refugees from Syria as government forces and Russian warplanes attack opposition-held areas.

Turkish government and aid officials said there was no sign yet of people reaching Turkey in large numbers, but that it was a matter of time until it happened.

"We are preparing our teams for a new wave," said Kerem Kinik, vice president of the Turkish Red Crescent.

"We have mobile kitchens, food packaged."

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