Paul Urey and Dylan Healey were reportedly captured early on Monday morning at a checkpoint south of the city of Zaporizhzhia in south-eastern Ukraine.
Paul’s mother Linda has now told Sky News that her son going missing has been her “worst nightmare”.
She said she had begged her son not to go but he had travelled to Ukraine a week and a half ago after returning home for leave.
Later, she added: “I told him not to go. I said please don’t go, he said: ‘Mum, I can’t live with myself knowing that people need my help to get to a safe place. I have to go’.”
On when she last spoke to him, she said: “I was on FaceTime with him up to 4am on Monday morning and that was it – gone.”
Ms Urey said her son messages and Facetimes her several times a day but now she is hearing “nothing”.
“Something’s wrong – they’ve got him, definitely. He would contact me if he could and he can’t,” she said.
She said she had been up since 3am watching the news and rewinding it to make sure she has not missed anything.
Asked what her son is like, she said: “Too caring, I wish he was a horrible person, to be honest. I know that sounds selfish but you don’t mess with the Russians. He’s too kind. I know nothing and I just want them to give him back.”
Ms Urey had previously released a statement saying her son has Type 1 diabetes and needs insulin.
Ms Urey said: “My family and I are extremely worried. We know my son Paul and his friend who was a humanitarian aid volunteer in Ukraine have been captured by the Russians.
“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 is also Type 1 diabetic and needs his insulin.
“We have asked the Presidium Network to help us and also the FCDO to help as well. We pray for him and hope he is safe.”
The Foreign Office said earlier it is urgently seeking more information following the reports of British nationals being detained in Ukraine.