PRZEMYSL, Poland — Their husbands and fathers stayed behind to fight. So the women and children fleeing war-ravaged Ukraine have mostly done so alone.

That makes their flight distinct from other mass refugee movements this century, prompting heightened concerns, from Poland’s eastern border to Germany’s capital, about human trafficking. European authorities and aid organizations are warning that criminals may seek to capitalize on the desperation of refugees, with more than 3 million leaving Ukraine since Feb. 24, according to U.N. estimates.

The International Organization for Migration on Wednesday said those concerns were based in reality, pointing to initial reports of traffickers exploiting the large-scale human displacement, including instances of sexual violence. The U.N. agency did not quantify the problem, noting that many cases go unidentified in the immediate aftermath of a displacement event.

Refugees from Ukraine are at heightened risk of exploitation because many are women, children and elderly people, some of whom are unaccompanied or separated, the agency’s director general, António Vitorino, warned in a statement. “These groups can be especially vulnerable to the risk of trafficking as they leave their homes unexpectedly and might have their usual family networks and financial security seriously disrupted,” he said.

Adding to concerns among some experts and aid workers are the cash incentives unveiled by governments, which were caught flat-footed by the Russian invasion and have depended on an outpouring of support from civil society. The Polish government is offering 40 zlotys a day, or just under $10, to residents who make space in their homes, while Britain is promising 350 pounds, or about $450, a month, while vowing “light touch” checks to guard against ill-intentioned hosts.

The issue of background checks has emerged as an early flash point of volunteer efforts, as governments and civil society weigh speed against safety. Another shield against possible exploitation is the existing Ukrainian diaspora across Europe, providing some refugees with networks of family and friends.

“It’s a complicated balance,” said Inga Thiemann, an expert on migration and human trafficking at the University of Exeter in England. “There is a risk associated with not doing checks on hosts — or not doing checks immediately — but leaving people to their own devices carries risks as well.”

The predicament illustrates the exigency of the deepening refugee crisis. And it represents a grim undercurrent to the display of hospitality at train stations and bus terminals over the past three weeks.

Averting the exploitation of “people who are at their absolute most vulnerable” is a priority for United States officials, too, said Samantha Power, administrator of the U.S. Agency for International Development. “That vulnerability poses a protection risk,” she said Tuesday on “CBS Mornings.”

At a train station in Lublin, Poland, a local aid coordinator said representatives from her human rights organization, Homo Faber, observed a woman loudly offering free places to stay and commenting on the beauty of the arriving women and children. The woman also said she could provide them with money if they gave her their passports. Homo Faber reported the woman to the police, said the coordinator, Karolina Wierzbińska, but she fled before authorities could question her.

“We have a lot of people who want to help,” Wierzbińska said of the droves of people who have driven to the border and reception centers with cardboard signs offering rooms in their apartments. “But we also have people who want to use these women.”

In response to the concerns, humanitarian groups are developing ad hoc methods of registering volunteers, and law enforcement is alerting arrivals about the possibility of danger.

In Berlin, where the city’s main train station is serving as an intake hub for refugees, the state government has added an alert to its page about arriving in Germany. “Please note that there may be criminals on the move at Berlin central station who want to profit from the situation of war refugees,” the note reads.

Volunteer groups coordinating their work on the messaging app Telegram are using those channels to raise awareness.

“Very important: if two men approach saying they have a place for young women with children to stay, especially if outside of Berlin, please go to coordinator and report this,” read one message in an arrival group in the southern part of Berlin. In the east of the city, a message recently went out highlighting extra precautions for unaccompanied minors. “Till further notice: NO unattended minor must leave the station privately in ANY way (NO private transport, NO private accommodation),” it read.

The problem is not systematic, said Sascha Langenbach, a spokesman for Berlin’s refugee affairs office, with no indication of organized criminal efforts to take advantage of the influx of people. And despite a handful of reports, neither Berlin nor federal police have confirmed instances of trafficking or scams. They say they have increased their presence at transit centers and use social media channels to inform people about potential risks. “We are taking indications of dubious offers very seriously,” said Stefan Strauss, a spokesperson for Berlin’s Senate Department for Integration, Labor and Social Affairs.

But aid workers say authorities are not adequately prepared to root out criminal attempts. Yasemine Acar, who helps organize an informal network called Berlin Arrival Support, said she struggled to persuade police to deploy sufficient personnel to patrol Berlin’s central bus station. Meanwhile, volunteer registration is an imperfect safeguard. With 15,000 sign-ups for her network alone, Acar said it would be easy for someone with bad intentions to put on a yellow vest and lend a hand.

Overnight hours are of particular concern. “It is not a safe situation for refugees arriving at the stations,” said Diana Henniges, a spokesperson for Moabit hilft, a refugee aid organization in Berlin.

In Poland, aid groups have been warning since the war’s start about human trafficking. To support government efforts to protect women, Homo Faber and other organizations developed a system to register drivers and use colored wristbands to demarcate approved volunteers to Ukrainian women and children. Homo Faber also created passport-size leaflets to distribute at the border with information about how to understand if someone is trustworthy.

Still, these efforts do not protect everyone, Homo Faber’s Wierzbińska said.

A 49-year-old man was arrested last week in Wrocław, in southwestern Poland, after he was accused of raping a 19-year-old Ukrainian woman whom he had promised shelter, according to police. If convicted, he could face up to 12 years in prison. Homo Faber and 41 other groupsoperating in Poland wrote a letter to the Polish government last week calling for more protections and official information for people fleeing Ukraine.

After visiting several border crossings and reception centers last week in Poland, Valiant Richey, a special representative at the Organization for Security and Cooperation in Europe responsible for combating human trafficking, found room for improvement, he said. Drivers were required to register their names and license plates, he said, but he didn’t see police performing criminal background checks or other types of questioning to weed out potential traffickers.

He recommended that governments implement uniform warnings and official instructions for border police to ensure that potential traffickers can’t slip through the cracks.

Richey is most concerned about trafficking not at the border but rather one or two transit stops later. “It increases the risk of human trafficking if you don’t put in safety measures,” he said, adding that all European countries, not just those neighboring Ukraine, must implement systems to keep women and children safe. “It’s beyond anecdotal at this point.”

Joe English, a UNICEF spokesman, said local and international organizations should be on the lookout especially for young children traveling by themselves. UNICEF, he said, has established centers to register children with social services and work with local organizations to support children.

“The issue that we face with trafficking and criminal networks are they are used to operating in the shadows,” said English, who has been spending time on both sides of the Poland-Ukraine border this week. “They do this in times of peace, and, especially, it is ripe for exploitation during times of crisis and chaos.”