Ukraine has the right to "project force" beyond its own borders, UK Foreign Secretary James Cleverly has said.
His comments come after Vladimir Putin accused Ukraine of "terrorist activity" after Moscow was hit by a drone attack.
Speaking after the drone strike, Mr Cleverly, who was in Estonia visiting RAF personnel deployed to the country, said: "Ukraine does have the legitimate right to defend itself.
"It has the legitimate right to do so within its own borders, of course, but it does also have the right to project force beyond its borders to undermine Russia's ability to project force into Ukraine itself."
The reserve Army officer went on to say that "legitimate military targets beyond its own border are part of Ukraine's self-defence. And we should recognise that".
Russian air defences stopped eight drones converging on Moscow, officials said on Tuesday, in an attack Russian authorities blamed on Ukraine.
The attack came as Russia continued its bombardment of Kyiv with a third assault on the Ukrainian capital in 24 hours.
Ukraine made no immediate comment on the attack on Moscow, which would be one of its deepest and most daring strikes into Russia since the Kremlin launched its full-scale invasion of Ukraine more than 15 months ago.
This comes after Ukraine's defence ministry said that its long-range Storm Shadow missiles, donated by the UK, have hit all Russian targets identified.
Responding to Mr Cleverly's comments, former Russian president Dmitry Medvedev has warned British public officials are now a "legitimate military target".
Mr Medvedev, deputy chairman of Vladimir Putin's security council, claimed the UK's support for Kyiv amounted to an "undeclared war" against Russia.
The Ukrainian authorities have denied launching the drone attack which hit Moscow on Tuesday, but there has been an escalation in incidents within Russia either by Kyiv's military or local groups opposed to the war.
In response to the Foreign Secretary, Mr Medvedev said: "The goofy officials of the UK, our eternal enemy, should remember that within the framework of the universally accepted international law which regulates modern warfare, including the Hague and Geneva Conventions with their additional protocols, their state can also be qualified as being at war.
"Today, the UK acts as Ukraine's ally, providing it with military aid in the form of equipment and specialists, ie de facto, is leading an undeclared war against Russia.
"That being the case, any of its public officials (either military, or civil, who facilitate the war) can be considered as a legitimate military target."
UK defence officials believe the incursions into Russia are causing the redeployment of Moscow's forces.
The intelligence briefing from the Ministry of Defence said: "Since the start of May 2023, Russia has increasingly ceded the initiative in the conflict and is reacting to Ukrainian action rather than actively progressing towards its own war aims.
"During May 2023, Russia has launched 20 nights of one-way-attack uncrewed aerial vehicle and cruise missile attacks deep inside Ukraine.
"Russia has had little success in its likely aims of neutralising Ukraine's improved air defences and destroying Ukrainian counter-attack forces.
"On the ground, it has redeployed security forces to react to partisan attacks inside western Russia." the briefing added.