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.

- June summit goal -

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 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.

Belgium on Tuesday committed to delivering 30 F-16 fighter jets by 2028 to help Kyiv battle Russia's invasion, at the start of a visit by Zelensky.

The Ukrainian leader was in Spain on Monday and then later in Portugal on Tuesday as part of a whirlwind tour of European capitals.

Spanish Premier Pedro Sanchez pledged one billion euros ($1.1 billion) in military aid and signed a security deal with Zelensky in Madrid.

Portugal said it would provide at least 126 million euros of aid this year and also signed a bilateral security agreement, Zelensky said in a post on social media.

The Ukrainian leader is trying to whip up support for a peace conference due to be held in Switzerland next month, as Kyiv has rejected any push to invite Russia.

On Tuesday, he urged US President Joe Biden to attend a June peace summit in Switzerland, accusing Putin of trying to derail the gathering.

"If (Biden) is not present, it will be just like applauding Putin: personally applauding and doing so standing," Zelensky said at a press conference with Belgium's Prime Minister Alexander De Croo.

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

Sweden said Tuesday that it was suspending plans to send Gripen jets to Ukraine, as its partners want to prioritise the delivery of US-made F-16s fighter jets to Kyiv.