Russian President Vladimir Putin said on 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.

Putin's warning came in response to calls within some Nato members as well as the alliance's chief Jens Stoltenberg to allow Ukraine to use their arms to step up attacks on Russian soil, in an attempt to end more than two years of fighting.

"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.

"They want a global conflict," he said. Western countries have long been wary of being deemed a belligerent in the Ukraine conflict as they provide support to Kyiv.

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".

Ukraine's top commander announced Monday that talks were being held with France on sending military instructors to the country.

"It is another step towards a serious conflict in Europe, towards a global conflict," Putin said.

"There are specialists there under the guise of mercenaries," he said, adding that "this was nothing new".

He said they would be "defeated" by the Russian army, as "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 were 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.

Ukrainian President Volodymyr 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 war 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. (AFP)