Live updates: follow the latest news on Russia-Ukraine
Shane Matthew, a former British Army sniper, arrived in Ukraine this week with a sense of duty, a cache of medical supplies - but no gun.
The 34-year-old former soldier, who served in Afghanistan, hopes to set up a field hospital in the Kyiv area with the help of the Ukrainian military to treat expected casualties as Russian forces advance on the capital.
He says he will take up arms if he is forced to defend himself. But with the legal position on foreign fighters unclear amid conflicting views from governments opposed to Moscow’s invasion, Mr Matthew says working as a field medic is the best way for him to serve.
“My motives for the medical direction were purely to prevent any chance of prosecution when I get home,” he said from the city of Vynnitsia, some 250 kilometres to the south-west of Kyiv. “Britain is not at war with Russia, fighting against the Russian military is still illegal.”
The dilemma faced by Mr Matthew is one being confronted by all the foreign volunteers travelling to Ukraine to join President Volodymyr Zelenskyy’s call to come to the aid of Ukraine.
Foreign Minister Dmytro Kuleba says 20,000 international volunteers have joined the fight against Russian forces, most coming from Europe.
They include Ben Grant, 30, the eldest son of a ruling party MP and former government minister, who spent more than five years in the Royal Marines.
Mr Grant, the son of Helen Grant, a former sport and tourism minister, is one of seven ex-servicemen who arrived in Ukraine over the weekend. He spent five years in the Marines.
He had been serving as a private security contractor in Iraq before deciding to travel to Ukraine after returning to join his family in the UK.
“I haven’t been sent, nothing to do with the government, nothing to do with my mother,” he told the Guardian newspaper at Lviv station on his way to Kyiv.
“Just wanna make that clear, completely off my own back, I decided to do this. I didn’t even tell my mum, but it is what it is.”
Russia has said it considers foreign fighters as mercenaries who would not be entitled to the protections guaranteed by the Geneva Convention for Prisoners of War.
Mr Grant, a former soldier, said he would rather take his own life than be captured, but added: “Let’s hope it doesn’t come to that”.
A UK government minister said on Wednesday British soldiers quitting their units to go on unauthorised missions to fight in Ukraine risked escalating the conflict with Russia if they were captured.
Four serving British soldiers had gone absent without leave, the Sun newspaper reported, with the Army scrambling to intercept them before they arrived in Ukraine.
They included a 19-year-old member of the Coldstream Guards, an infantry regiment with an added ceremonial role of protecting royal palaces in the UK.
Grant Shapps, the UK transport secretary, said British soldiers heading to Ukraine to fight were creating a “dangerous situation”.
“You cannot go and fight if you’re in the British Army, you cannot just get up and go and fight,” he told broadcaster ITV.
Asked what Russia might do if a British soldier was captured or killed, Mr Shapps said: “There’s a big difference between Britain sending its army in and some people who are breaking with our law and going to do it.
“But, clearly, this is a dangerous situation. And clearly, we want to make sure that the assistance we’re providing is done in an official way – like the anti-tank missiles that we provided prior to and during this conflict, and like the 22,000 Ukrainians that we’ve trained.”
The British government has sent mixed messages on the issue with Foreign Secretary Liz Truss facing criticism for saying she would back Britons going to Ukraine to fight Russia.
The chief of the defence staff, Tony Radakin, said the move would be “unlawful as well as unhelpful” for people to go to fight.
British citizens travelling to fight could potentially be prosecuted under the 1870 Foreign Enlistment Act, but the UK has a long history of applying the law "very selectively," said Nir Arielli, associate professor of International History at the University of Leeds in an article for The Conversation.
Ukraine’s government has set up its own website urging people to sign up with seven steps to sign up to a “voluntary contract-based” military service in the armed forces of Ukraine.
It included providing details of military service and an interview at an embassy. But many volunteers are heading straight to the border with Poland where volunteers are reportedly being met by officials in preparation for heading to battle.
The Ukrainian Ambassador to the UK, Vadym Prystaiko, warned against some Britons travelling to his country because it might divert from the war effort.
He said some with medical skills could be helpful on the ground, while others could not be stopped from travelling.
“We are not encouraging people to go but at the same time I can’t stop most of them to go there,” the ambassador told MPs on the Home Affairs select committee.
“People are gathering, some are coming with particular help like tactical medical stuff who are brave people and know what to do immediately, people [who] actually can be helpful on the ground, helping civilians, getting them out of these areas,” he said.
“Some people are coming with logistical support, brave enough to use their own trucks and buses to go… we have to stop them actually from doing it because we don’t want to take care of yet another citizen of UK on our territory instead of fighting.”
Mr Matthew flew to Warsaw, Poland, and crossed the border without problem. He did not sign any undertakings as he was on a humanitarian mission.
He said he was hunkered down in Vynnitsia, but hoped to move on Thursday as part of a humanitarian convoy.
He is liaising with Ukrainian military units and would set up alongside them to receive a level of protection and help if he needed to leave in the face of a Russian advance.
He says those without military experience should stay in the UK as they represented a danger to themselves and those around them. “Send aid, donate but stay at home,” he said. “War is not a game, is not Call of Duty [an online game]”.
He added: “I’m medical, not a foreign soldier, I’m not fighting. I’m not overly concerned about that [being captured].”
“That being said they’re not really sticking to the rules of war, or the Geneva Convention, at the moment, so I may have to cross that bridge when I come to it.
“But it doesn’t faze me.”