Sex traffickers are taking advantage of young women and unaccompanied children fleeing the war in Ukraine, police and volunteers in Poland have warned.

As more than a million refugees flood westwards to escape Vladimir Putin's invading troops, a darker side to their welcome in central Europe has emerged.

Police on the border with Ukraine have made multiple arrests in the area of suspected traffickers preying on vulnerable woman and children with the promise of a lift and accommodation, sources told The Telegraph.

The authorities are understood to be particularly concerned about gangs seeking to lure vulnerable refugees to Germany for sexual exploitation.

Overwhelmed by the human tide crossing into Poland, officers are asking humanitarian volunteers at the Medyka checkpoint to demand to see proof of identities for men offering lifts to young women.

Tom Bell, one British volunteer, said: "A lot of desperate Ukrainins have gone on to Facebook or Telegram trying to hook up with people in Europe who can help them.

"Then, in a short space of time, they're getting picked up in a car by someone they've never met and don't know.

"The police have put me and other volunteers here on alert - they want us to check people's IDs to see who they are if they're picking up younger women, and for kids.

"We've been told it's a sex trade thing."

Mr Bell said vigilance had increased since last Thursday.

At both railway terminuses and road border crossings, there is a semi-formal network directing refugees to onward transport and accommodation.

However, there is nothing to prevent unaffiliated people stepping in to intercept the often exhausted new arrivals.

Last week, Save the Children warned that at least 400,000 young people were on the move across eastern Europe, putting them potentially at risk of abuse.

The organisation estimated that more than 40 per cent of refugees who have fled the fighting to Poland, Romania, Moldova, Hungary, Slovakia and Lithuania are children.

Even before Putin invaded Ukraine, Polish authorities have grown increasingly experienced in countering human smuggling gangs in the last year. Alexander Lukashenko, the Belarusian president, has deliberately channeled migrants from the Middle East into the European Union.

A police source at Medyka said: "We are concerned about the exploitation of children and women coming over the border.

"We know of a few examples of people who have tried it, and some people have been arrested already."

Poland has also stepped up its efforts to ensure children entering the country without an obvious relative are legally accounted for.

All-night courts have been taking place to approve legal guardianship of adults offering to look after non-related children once in Poland.

Mr Bell was last week assigned guardianship of Maks Kowalenko, a nine-year-old Ukrainian boy - the nephew of a friend he plays rock music with - whom he travelled into the war zone to rescue.

The 37-year-old, who lives in southern Poland, said: "The Polish border guards sat Maks and me in a room almost all night while they spoke to his mother to make sure I was who I said I was.”

A father already, he said: "I have to say I didn't expect to be adopting a child, but I've said to Maks' family that in a worse-case scenario that the war doesn't end and he can't go back, he can live with my family indefinitely."

Lauren Agnew, human trafficking policy expert at the social policy charity Care, said the conflict in Ukraine was “hugely alarming” for those working to combat human trafficking. She warned governments across Europe to be “mindful of the added dangers we now face”.