STORY: The non-profit Presidium Network said the two men, both civilians, were working independently as part of a project in Ukraine to provide food, medical supplies and evacuation support.
Dominik Byrne, Presidium Network's co-founder, said the men had gone missing on Monday (April 25) after going into Russian-held territory where they were planning to help evacuate a woman.
Three hours after they were supposed to arrive at her home, the woman said she had been questioned by Russian soldiers about two men they had picked up, Byrne said.
One of the men has type 1 diabetes and needs daily insulin to manage the condition, Byrne said.
There was no immediate comment from the British foreign ministry but British trade minister Anne-Marie Trevelyan told Sky News: "The foreign office is doing all it can to support and identify these two people," .