The UK's charity watchdog is urging aid groups to make safeguarding issues a priority to help protect Ukrainian refugees from abuse.
More than 4.9 million refugees have arrived in Hungary, Poland and Slovakia since Russia invaded Ukraine in February.
The UK's Charity Commission said that while aid groups operating at the borders were doing "vital" work, they need to be alert for attempts to use them as a guise for committing abuse.
The commission said it would take action against charities which have not made appropriate checks.
“The situation in Ukraine continues to change rapidly, with many Ukrainians becoming displaced and seeking the shelter and safety of charities operating along Ukraine’s borders and in the neighbouring countries,” the Charity Commission said.
“We recognise the vital work charities are delivering, here and in the region, to support those affected by the crisis. Through our collaboration with the Foreign Commonwealth and Development Office (FCDO), we are aware that criminal groups are operating in the region.
“Charities need to be alert to the risks that some individuals may try to use the cover of charity for the purpose of sexual exploitation and abuse and sexual harassment of those in need of assistance.
“All trustees must take reasonable steps to protect from harm people who come into contact with their charity – this is a fundamental part of fulfilling your trustee duties and operating as a charity for the public benefit."
The UN has already told of criminal gangs trying to exploit women and girls fleeing the war in Ukraine.
"For predators and human traffickers, the war in Ukraine is not a tragedy," UN Secretary General António Guterres said on Twitter.
"It's an opportunity – and women and children are the targets."
The commission is urging groups to review safeguarding policies and procedures regularly to ensure they remain fit for purpose as the situation on the ground evolves.
It says staff and volunteers need to be vetted to ensure they are suitable and legally able to work for charities.
It also advises using the Misconduct Disclosure Scheme when hiring to help protect charities from those who pose a safeguarding risk.
"If your charity is responding to the crisis in Ukraine by working with partners, you must make sure that any grant recipient or partner body is suitable and they must have appropriate safeguarding procedures in place," the commission said.
"Where a registered charity supports, or works closely with overseas partners or not-for-profit organisations that are not registered with the commission, we will hold the registered charity to account over the suitability and management of that relationship – including its supervision of safeguarding risks.
"Where allegations or incidents do arise, it is important that charities respond quickly and handle the situation appropriately. Charities should ensure that support is available to victims and survivors. They must also report to all relevant agencies and regulators as appropriate."