The men, aged 28 to 48, were doing volunteer work and were last seen on Friday on their way to Soledar, where fighting has been fierce in recent days.
There has been no contact with them since then.
A Foreign Office spokesman said it was “supporting the families of two British men who have gone missing in Ukraine”.
The police department in the city of Bakhmut said they received a missing person’s report at 17:15 local time on Saturday, while appealing for any information that could help locate the two men.