President Vladimir Putin said Tuesday that there would be "serious consequences" if Western countries allowed Ukraine to use their weapons to strike targets in Russia, as sought by Kyiv.
The warning came as French President Emmanuel Macron said Kyiv should be allowed to "neutralise" Russian military bases from where Kremlin troops are firing missiles into Ukraine, and as President Volodymyr Zelensky urged the world not to tire of the war.
Putin's comments came after some NATO members as well as the alliance's chief Jens Stoltenberg have called to allow Ukraine to use their arms to step up attacks on Russian soil, after more than two years of war.
"This constant escalation can lead to serious consequences," Putin said during a visit to Uzbekistan.
"In Europe, especially in small countries, they should be aware of what they are playing with," he said, noting that many European countries had "small territory" and a "dense population".
"And this factor, which they should keep in mind before they talk about striking deep into Russian territory, is a serious thing," he said.
He added that even if Ukraine's forces carried out the strikes, responsibility for them would lie with Western suppliers of the weapons.
Putin also said that while he believed Western military instructors were already in Ukraine operating undercover as mercenaries, any move by countries to send them officially would be another "escalation" and "another step towards a serious conflict in Europe, towards a global conflict".
Ukraine's top commander announced Monday that talks were being held with France on sending military instructors to the country.
Putin warned that "we will do what we think is necessary regardless of who is on the territory of Ukraine".
EU foreign affairs chief Josep Borrell said after a meeting of EU defence ministers in Brussels that European countries remained split on sending military instructors to Ukraine.
Countries including Germany oppose taking a step they fear could potentially drag them closer to direct conflict with a nuclear-armed Russia.
EU countries have trained 50,000 Ukrainian troops outside the war-torn country under a bloc-wide mission set up in 2022.
Zelensky has been pressing Western allies to provide longer-range missiles and other material for striking deeper into Russia in a bid to cripple its military and industrial capacity.
With the Russian assault now in its third year, Ukraine is also pleading for more weapons for its outgunned and outnumbered troops, notably seeking help to address its lack of air defence systems.
So far Kyiv's partners have demanded that their arms not be used to attack Russian soil, which Kyiv forces have instead been doing with local-made explosive drones.
But France's Macron said that "We think that we should allow them to neutralise military sites where missiles are fired, from where... Ukraine is attacked," but that "we should not allow them to touch other targets in Russia, and obviously civilian capacities".
Zelensky, on a European tour to keep arms and support flowing to his embattled country, urged the world not to get tired of the war, saying it would play into Russia's hands.
"It is very important for Ukrainians that the world does not get tired... that the world understands that it cannot get tired of the war launched by the aggressor," Zelensky said in Portugal during a tour of several European capitals.
"Otherwise there will be no justice, otherwise the world will change, otherwise the world will be changed by people like Putin," he said.
Zelensky is also trying to whip up support for a peace conference due to be held in Switzerland next month, without Russia, and on Tuesday urged US President Joe Biden to attend.
Putin is "very scared" of the peace summit aimed at agreeing the terms needed to end the conflict, Zelensky said, adding that "He has been trying to thwart this summit and continues to do so".
Kremlin spokesman Dmitry Peskov said any such conference was "hopeless" without Russia's participation.
