Dylan Healy, 21, from Cambridgeshire, was reportedly caught along with another British man, Paul Urey, on Monday at a checkpoint south of the city of Zaporizhzhia in south-eastern Ukraine.
Mr Healy, a chef from Huntington, was trying to help people flee areas of the country under Russian occupation when the pair were captured at a military checkpoint, said the UK non-profit organisation Presidium Network.
The Foreign Office said they're working to find out more and support the two individuals, but friends say they're worried.
Allan Moore, who has been friends with Mr Healy for almost three years, said he can't believe the news.
"It's quite shocking to be honest with you. I don't know how to react to it," he said.
"I'm still in the back of my mind thinking this a joke and he's going to be alright, but at the same time, seeing the way Putin and some of the Russia's dealt with people already, it's quite inhumane. Hopefully he's okay or manages to get a way out."
Mr Moore told ITV News Anglia that he used to play football with Mr Healy for the Huntingdon Rangers Sunday League team.
He said he believes Mr Healy went to Ukraine "to try and help and make a difference" and hopes he will return home safely.
Dominik Byrne, co-founder of Presidium Network, said the men were driving to try to evacuate "a lady and two children".
He said the two men had been operating on their own in the war zone and had not been associated with any aid group.
Mr Bryne said their contacts have not heard from them since Monday morning, and the woman who they had been trying to help leave Ukraine received "strange text messages".
Her house was later stormed by Russian forces who interrogated the family, making her husband lie on the floor and demanding to know about their involvement with "British spies", Mr Byrne said. The family later escaped to Poland, he said.
It comes after a British military veteran was killed in Ukraine and another is missing while fighting Russian president Vladimir Putin’s forces. Tributes were paid to Scott Sibley the first British national known to have been killed in Ukraine.
It is understood Mr Sibley was fighting with Ukrainian forces.