It would be acceptable for Ukrainian forces to use Western weapons to attack military targets on Russian soil, a UK defence minister has said.
James Heappey said strikes on supply lines were a “legitimate” part of war.
The UK announced it will give Ukraine a small number of anti-aircraft vehicles.
Russia’s defence ministry has accused the UK of “provoking” Ukraine into attacking Russian territory, while foreign minister Sergei Lavrov said Nato is engaging in a proxy war.
Mr Lavrov also said weapons delivered by the West to Ukraine would be fair targets.
Western countries have donated hundreds of millions of pounds of military aid to Ukraine since Russia launched its invasion back in February and Nato and European Union officials are meeting in Germany to discuss further military assistance.
It comes as Russia has focused its forces on the eastern Donbas region of Ukraine, with the UK’s Ministry of Defence reporting the city of Kreminna, in Luhansk, has fallen.
Russia has accused Ukraine of attacking targets within its territory, including an oil depot in Belgorod, but Ukrainian forces have not confirmed any strikes.
Asked about this on BBC Radio 4’s Today programme, Mr Heappey said: “The question is, is it acceptable for our weapons to be used against legitimate Russian military targets by the Ukrainians?
“Firstly, it’s Ukrainians that take the targeting decision, not the people who manufacture or export the kit in the first place. And secondly, it is entirely legitimate to go after targets in the depth of your opponents to disrupt their logistics and supply lines.”
The armed forces minister added that it was also a “perfectly legitimate” part of war for Russian forces to be striking targets in western Ukraine to disrupt Ukrainian supply lines as long as they avoided targeting civilians – “which unfortunately they have not taken much regard for so far”.
On Monday Mr Lavrov claimed the West was “pouring oil on the fire” by providing Ukraine with firepower and repeated warnings that the conflict could lead to a third world war.
But Mr Heappey said the West had been very careful about the donation of weapons to Ukraine which was not a “purely Nato endeavour”.
He said Russia had been saying it was in a conflict with Nato before the war began – “it’s nonsense and [Mr] Lavrov knows it”.
Mr Heappey added he thought the chances of a nuclear war were “vanishingly small”, with no-one wanting such an outcome to the conflict.
Russia’s defence ministry has also accused the UK of provoking Ukraine into attacks on facilities located in Russian territory.
It added that Russia would give a “proportional response” to any such attempt, the Interfax news agency reports.
“The Russian army is ready to deliver retaliatory strikes against decision-making centres in Kyiv using high accuracy weaponry,” the ministry said.
For months Ukrainian President Volodymyr Zelensky has been asking Western allies for more weapons to fight off the Russian invasion.
On Monday, Defence Secretary Ben Wallace announced the UK would gift a small number of Stormer vehicles fitted with launchers for Starstreak anti-aircraft missiles to give Ukrainian forces “enhanced, short-range anti-air capabilities both day and night”.
He also told the Commons around 15,000 Russian troops have been killed since Russia launched a full-scale invasion of Ukraine, while 2,000 of its armoured vehicles had either been destroyed or captured.
US Defence Secretary Lloyd Austin is chairing the meeting of defence ministers at the US Air Force base at Ramstein in Germany on Tuesday.
Ahead of the gathering there are reports suggesting Germany would pledge to supply anti-aircraft systems to Ukraine.
The UN Secretary General Antonio Guterres is due to meet Vladimir Putin during a visit to Russia, having already met Mr Lavrov in Moscow.
During a news conference, he said the priority was to minimise suffering in Ukraine and end the war as soon as possible.
Talks are expected to focus on humanitarian corridors out of the besieged port city of Mariupol.
Mr Guterres is expected to meet President Zelensky later in the week.
UK Foreign Secretary Liz Truss told MPs that the government had concerns “about an international security architecture that has Russia as one of the permanent members of the [UN] security council”, adding that Russia had used its veto “as a green light for barbarism”.
She said part of the UK’s response had been working closer with allies like the G7 and Nato “because we simply haven’t seen enough at a UN level”.