Russian forces have captured a British man fighting with the Ukrainian army in Russia's Kursk region, Russia's RIA news agency cited a security source as saying after a video of a captive emerged over the weekend.

In remarks published on Sunday, the source said:

"A mercenary from Great Britain, who called himself James Scott Rhys Anderson, was captured. He is now giving evidence."

The UK Foreign Office said it was working to support the man as well as his family following reports of his detention.

In a video posted on unofficial pro-war Russian Telegram channels on Sunday, a young bearded man wearing military clothing, his hands apparently tied behind his back, says in English that his name is James Scott Rhys Anderson and that he formerly served in the British Army.

State news agency Tass quoted him as saying that he had served as a signalman in the British army for four years and then joined the International Legion of Ukraine, formed early on in Russia's nearly 3-year-old war against its neighbour.

The International Legion for Defence of Ukraine was created at the request of Ukrainian President Volodymyr Zelensky shortly after Russia’s full-scale invasion in February 2022.

The Legion is a unit of Ukraine’s Ground Forces that consists mostly of foreign volunteers.

Apart from the Legion, Ukraine recruits foreigners to other units of its army, filling squads, companies and battalions.

Early on in the war, Ukraine authorities said over 20,000 people from 52 countries came to Ukraine to help it defend itself against Russia's aggression.

In this photo taken from video, Russian servicemen operate a Russian Uragan self-propelled multiple rocket launcher in an undisclosed location. – Russian Defence Ministry Press Service via AP

Ever since, the numbers of foreign fighters in the ranks of the Ukrainian military have been classified.

In Ukraine, Anderson reportedly served as an instructor for Ukrainian troops and was deployed to the Kursk region against his will.

In the video, the man reportedly said in English that he doesn’t want to be “here”.

The reports coming out of Russia couldn’t be independently verified, but if confirmed could be one of the first publicly known case of a Western national captured on Russian soil while fighting for Ukraine. It was not clear when the video was filmed.

The UK Embassy in Moscow said officials were "supporting the family of a British man following reports of his detention” but provided no further details.

The soldier’s father, Scott Anderson, said his son’s Ukrainian commander had informed him that the young man had been captured.

He said he tried to convince his son not to join the Ukrainian military, and now he fears for his safety.

“I’m hoping he’ll be used as a bargaining chip, but my son told me they torture their prisoners and I’m so frightened he’ll be tortured," he said.

Britain's Foreign Secretary David Lammy, when asked about Anderson, told reporters in Italy: 

"I have been updated about that development in the last couple of days. Of course, we will do all that we can to offer this UK national all the support we can."

On the battlefield, Ukrainian forces are straining to hold at bay a push by Russia's bigger army at places in the eastern Donetsk region.

Russian forces recently have gained ground at “a significantly quicker rate” than they did in the whole of last year, according to the Institute for the Study of War, a Washington think tank.

The Russians have detected and are exploiting weaknesses in the Ukrainian defences, it said in an analysis.

The war surpassed 1000 days last week, and the milestone coincided with a significant escalation in hostilities.

Meanwhile, Ukraine’s air force said Russia is adapting its drone tactics, as it fired 145 Shahed drones at Ukraine.

Aftermath of Russia's missile attack on Kharkiv. – AP

Russia has started launching drones during the day, whereas in the past most drone attacks occurred during the night, the air force said.

Andrii Kovalenko, the head of the National Security Council’s Counter-disinformation Centre, said earlier this month that the Russians were looking to conserve their stocks of more destructive but more expensive missiles and also terrorise civilians.

The air force said it stopped almost all the drones before they struck.

But a morning missile attack on downtown Kharkiv, Ukraine’s second-largest city in the northeast, injured at least 23 people, according to Mayor Ihor Terekhov.

He said the attack on a densely populated residential area was carried out by a modified surface-to-air S-400 missile.