Russia's ongoing attacks against Ukrainian infrastructure could constitute a war crime and Vladimir Putin "must be held to account", western officials said on Tuesday.
Putin's army has been attempting to destroy Ukraine's energy and water sources in a wave of drone and missile strikes for several weeks.
Some areas are without electricity, water or heating following the onslaught, as winter temperatures plunge to -4C.
"It would be for a competent court to determine whether these constituted war crimes, but intentionally directing attacks against civilians and civilian objects does constitute a war crime," a western official said.
"Those responsible must be held to account."
They added that the number of attacks on infrastructure showed Putin's "increasing desperation" and were a "recognition that his military intervention is failing".
"The object is to break the spirit of the Ukrainian people," they said. "But if [Putin] calculates that that is the result, he is mistaken. "The resolve has only been strengthened.
"The UK will continue to support [Ukrainians] in repairing their infrastructure, improving their defences and sustaining them in their legitimate legal right to self-defence."
Ukrainian President Volodymyr Zelensky travelled to an eastern city near the front line after two new strategic sites inside Russian territory were reportedly hit by drone attacks.
A fire blamed on a drone strike broke out at an airport in Russia's southern Kursk region that borders Ukraine, the region's governor said.
In a second incident, an industrial plant 50 miles from the Ukrainian border was reportedly also targeted by drones, apparently missing a fuel depot at the site.
The strikes were carried out a day after Moscow blamed Kyiv for unprecedented strikes on two air bases deep inside Russia.
British defence chiefs said Putin’s generals will see attacks on air bases deep inside Russia as the “most strategically significant failures of force protection” in his war against Ukraine.
They believe that the Kremlin’s military top brass will try to blame and punish some officers for the failure to stop the strikes on the Engels Airbase, in Russia’s Saratov province and at Dyagilevo airfield near Ryazan, south-east of Moscow on Monday.
Two Tu-95 BEAR heavy bombers were reportedly damaged at Engels and three people killed when a fuel tank exploded at Dyagilyaevo.
In a breifing the Ministry of Defence in London said: “If Russia assesses the incidents were deliberate attacks, it will probably consider them as some of the most strategically significant failures of force protection since its invasion of Ukraine.
“The sites are much deeper inside Russia than previous similar explosions: Engels is over 600km from Ukrainian-controlled territory.”
They added: “Engels is the main operating base of Russia’s Long Range Aviation (LRA) within western Russia and is home to more than 30 heavy bombers.
“These aircraft contribute to Russia’s nuclear deterrent and have also frequently been used to launch conventional cruise missiles at Ukraine.
“The Russian chain of command will probably seek to identify and impose severe sanctions on Russian officers deemed responsible for allowing the incident.”
The attacks on the air bases are believed to have been carried out by Ukrainian forces using drones.
Ukraine did not directly claim responsibility for the attacks.
Marking Ukraine’s armed forces day, Mr Zelensky travelled to the eastern Donetsk region and vowed to push Russian forces out of all of Ukraine’s territory.
He said in a video address to Ukrainian forces from the city of the Sloviansk, a key Ukrainian stronghold in the east: “Everyone sees your strength and your skill.
“I’m grateful to your parents. They raised real heroes.”
