ITV News Correspondent Rebecca Barry reports from Lviv about the two British aid workers who have gone missing in Ukraine
UK non-profit organisation Presidium Network said Paul Urey and Dylan Healy were captured on Monday at a checkpoint south of the city of Zaporizhzhia in south-eastern Ukraine.
Mr Urey, 45, from Manchester, was described by Mr Bryne as a "family man with children". His mother, Linda, said the family "were extremely worried" about Mr Urey, particularly as he is diabetic and needs insulin.
Mr Healy, who is in his 20s, a chef from Huntingdon, Cambridgeshire was driving the car when the pair were captured by the Russian military at a checkpoint, the Presidium Network said.
The Foreign Office said it is urgently seeking more information.
Dominik Bryne, co-founder of Presidium Network, said the men were driving to try to evacuate "a lady and two children".
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".
He provided screengrabs of WhatsApp messages Mr Uley had sent to his contacts in the hours before his capture, including code words used for security. He also sent a grid reference showing his and Mr Healy's position.
His phone then went offline and once it came back online, suspicious Viber messages were sent from his number that have been verified as fake. These messages were not written in their natural style and failed to issue code words for security.Russian troops later stormed the house of the woman the men were attempting to help evacuate. The soldiers interrogated the family, making the husband lie on the floor as the Russians demanded to know about their involvement with "British spies", Mr Bryne said. The family later escaped to Poland, he said.
A friend of Mr Healy, Allan Moore, told ITV News: "In the back of my mind, I'm still thinking 'it's a joke, he's going to be alright'. "But at the same time, we've the way that Putin and some of the Russians have dealt with people already, it's quite inhumane. "Hopefully he's OK, or he manages to get his way out of it."
The Presidium Network said the men had been operating on their own in the war zone and had not been associated with any aid group.
Linda Urey said in a statement: "My family and I extremely worried. We know my son Paul and his friend who was humanitarian aid volunteer in Ukraine has been captured by the Russians."He was out there on his own accord. We want everyone’s support to bring son home and pray he is safe."My son Paul Urey 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."
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Mr Bryne said: “Basically what needs doing is two things – we need to put pressure on the government to take this case seriously, and try, through their networks to verify this, but also to kind of help us find these people.
“But also because I know diplomatic channels are completely broken down, we use these tactics to find people by having it very public and putting pressure publicly on Russia to determine that they have got these two people and that they’re safe and well.
“And we want to tell the Russians basically that these aren’t spies. These aren’t military people. These are just humanitarian workers who got caught in a bad situation.”
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.
"We can confirm that a British national has been killed in Ukraine and are supporting their family,” the Foreign Office said on Thursday.
ITV News Correspondent Dan Rivers on what we know about the situation
"We are aware of a British National who is missing in Ukraine and are supporting their family," the department added. "We are urgently seeking further information."
Following the death and disappearance of the British citizens, Foreign Secretary Liz Truss announced the government will send war crimes experts to help Ukraine with its investigations into atrocities committed by Russian invaders.The Foreign Office said the team of experts, due to arrive in neighbouring Poland in early May, will support the Ukrainian government in gathering evidence and prosecuting war crimes and will include experts in conflict-related sexual violence.
The FCDO has advised against travel to Ukraine and in March warned any who do could expect to be investigated on their return. Service personnel are banned from heading to Ukraine to join the fight.
A 25-year-old Dane was allegedly killed in Mykolajiv on April 26 while fighting with the International Legion Ukraine, a unit for foreigners who want to join the fight against Russia, according to Danish broadcaster TV2. The man has not been named.