London police's war crimes unit on Friday launched an appeal for information about alleged atrocities in Ukraine, as part of a probe by the International Criminal Court.
The ICC's chief prosecutor on Wednesday announced an "active" investigation into possible war crimes in Ukraine, after his office received the backing of 39 countries.
Metropolitan Police detectives said they wanted "anyone in the UK who may have direct evidence of war crimes in Ukraine" to get in touch.
The force's specialist War Crimes Team has national responsibility for investigating anyone under UK jurisdiction over alleged war crimes, crimes against humanity, genocide or torture around the world.
Counter Terrorism Command chief Richard Smith said detectives were looking for any evidence from November 21, 2013 to the present day.
"This could be in the form of direct messages, images or videos that friends or relatives here in the UK have been sent by those in Ukraine," he added.
"Or it could be somebody who was previously in Ukraine and who may have witnessed or even been a victim of a war crime and has since travelled to the UK."
He added: "To be clear though, we specifically want to hear from people who are now here in the UK, and who have a direct link to any relevant information or evidence.
"This is not about the general public highlighting or telling us about things they might have simply come across or seen in the media, online or on social media."
The evidence gathered may then be shared with the ICC. Officers at UK borders have been told to make new arrivals from Ukraine aware of the appeal.