The family of a British man fighting in Ukraine said he has told them he will have to surrender to Russian forces.
Aiden Aslin, from Newark in Nottinghamshire, has been fighting in Ukraine since he moved there in 2018, and became a marine in the country's military.
His unit has recently been defending the besieged city of Mariupol, which has been heavily bombarded by Russia.
His mother Ang Wood said her son told her he had no choice.
"He called me and said they have no weapons left to fight," she told the BBC.
"I love my son, he is my hero - they put up one hell of a fight."
Mr Aslin is a former care worker who previously fought with Kurdish armed units in Syria against so-called Islamic State.
He has a Ukrainian fiancée and now has dual citizenship.
Ms Wood said her son "sounded OK" when she spoke to him, but called for the UK government to provide more support to Ukraine.
"Boris [Johnson] needs to take [Vladimir] Putin down," she added.
Brennan Philips, a friend of Mr Aslin who also spoke to him by phone, said he sounded "strong and in good spirits".
"He called me and said 'we have no food, no supplies, no ammo, we're completely surrounded, we have to surrender'," he said.
"Aiden was very well aware of what was going on, very calm.
"They can't get out, they can't fight back so they had no choice.
"I'm sure if they had a bullet left they would have shot it."
By Emma Vardy, BBC News correspondent
Aiden had been due to get married last week to his Ukrainian fiancée.
But his unit had become increasingly cut off as Russian forces bombarded the city of Mariupol in recent weeks, and Aiden's family and loved ones have had an extremely anxious wait as the situation deteriorated.
Aiden had been in communication periodically, sometimes sending video updates of himself to friends to post on social media, or just checking in to let his family know he was safe.
But the news that he was having to surrender has been a huge blow.
There are now Russian reports coming out claiming the 36th Marine Brigade have surrendered, which Aiden's family understand is his unit, though his family have heard nothing further since his initial phone calls.
Aiden is no newcomer to the perils of conflict. Before joining the Ukrainian forces, he had previously left the UK to fight in Syria against IS, fighting with the Kurdish armed group the YPG.
To the Kurds he is a hero, although he and many others like him faced questioning by counter-terrorism police when they returned to the UK. Most cases were eventually dropped.
Now what has happened to him in Ukraine is uncertain. When Aiden spoke to his family it appeared his unit were communicating with the Russians in what appeared to be preparation for a negotiated surrender.
The key question is what would happen if he were to be taken as a prisoner of war? His friends have appealed on social media for Aiden and his unit to be treated in accordance with the Geneva conventions.
The UK's Foreign Office is working to verify reports of UK nationals like Aiden involved in the fighting. But given the situation, it is likely that the Foreign Office's powers will be quite limited.
Russia has claimed more than 1,000 Ukrainian soldiers have surrendered in Mariupol but Ukraine says the city is still standing despite being under siege.
Mariupol's mayor said about 21,000 civilians had been killed with another 100,000 awaiting evacuation.
The city's fate is likely to be critical for the next phase of the war.
In Russian hands, it would provide control of a clear swathe of territory connecting Moscow's two fronts in the south and east.
Additional reporting by Liam Barnes.