Paul Urey and Dylan Healy were attempting to pass through a check point near the southern Ukrainian city of Zaporizhzhia on Monday as they tried to rescue a family of a mother and two children from a nearby village when they were believed to have been taken by Russian forces.
Linda Urey said her son, who is a type one diabetic who relies on insulin, would usually message her up to 20 times a day while he was in Ukraine, but said she had lost contact with him. She told how she had begged him not to travel to Ukraine.
"I said ‘please don’t go’,” she said.
"He told me ‘I can’t live with myself knowing people need my help to get to safety, I have to go’.”
Mr Urey, 45, from Manchester, is understood to have children.
Ms Urey added: “He was out there on his own accord. We want everyone’s support to bring my son home and pray he is safe. My son, Paul Urey, is also type one diabetic and needs his insulin.”
Mr Healy, 22, a kitchen manager at a hotel chain, is from Cambridgeshire. He was believed to be driving a car when the two were captured.
The pair were believed to have been working independently, but were connected with charity Presidium Network, based in England.
The Foreign Office said it was urgently seeking more information.
On Thursday it emerged a British man, Scott Sibley, had been killed while he was believed to be fighting with the Ukrainian military, while another was missing.
Two other British men, Aiden Aslin, 28, and Shaun Pinner, 48, were captured earlier this month while fighting in the south-eastern city of Mariupol. They later appeared on Russian state TV with apparent facial bruising.
Presidium Network's founder Dominic Byrne said the men had negotiated for six hours to be able to get through the last checkpoint and into Russian controlled territory, where they were detained.
He said the men were trying to carry out evacuations of Ukrainian citizens themselves with some “small, unverified support from the UK” – and had been in touch with his charity so that someone had their details “in case of emergency”.
After losing contact with the men on Monday, the woman who was trying to be evacuated then received some unusual text messages and screen shots from the men’s phone, which Mr Byrne said were “not consistent” with the way they had communicated before.
Her statement is that soldiers detained her husband on the ground and were shouting, saying ‘why do you know these two British people’? You shouldn’t be talking to them, we think they’re spies’.
"From that, we can confirm that they are 100 per cent likely to be under Russian capture.”
Mr Byrne said: “She got a text message saying they were five minutes away, then two hours later her house was swarmed by Russian soldiers.”