The Kremlin claimed Donald Trump was showing signs of "emotional overload" after he called Vladimir Putin "absolutely crazy" following Moscow's largest aerial assault on Ukraine.
The US president said on Truth Social on Sunday that "something has happened" to Putin, after Russia killed 13 in Ukraine with 367 drones and missiles. "He has gone absolutely crazy," Trump said. "Needlessly killing a lot of people."
Dmitry Peskov, Putin's spokesman, said the comments were "connected to an emotional overload of everyone involved".
Germany's chancellor, Friedrich Merz, meanwhile said that Ukraine's allies had removed all range limits on supplied arms, amid reports he would give Kyiv Taurus missiles.
Trump's comments followed Russia's largest combined aerial attack since its full-scale invasion of February 2022. At least 13 people were killed and dozens injured in Ukraine during the night between Saturday and Sunday after Russia fired 367 drones and missiles.
Between Sunday evening and Monday morning, Russia launched 355 drones against Ukraine, killing 10. The Ukrainian air force said it was the largest attack yet conducted with drones alone.
Peskov said the latest aerial assaults were a response to Ukrainian attacks on Russia's "social infrastructure".
The Russian defence ministry said that air defence systems destroyed 20 Ukrainian drones over several Russian regions.
Volodymyr Zelensky, Ukraine's president, said on Sunday there was no "military sense" to Russia's aerial attacks - rather they were "an obvious political choice... by Putin, a choice by Russia... to continue the war and destroy lives."
In an apparent response to the Russian attacks over the weekend, German chancellor Merz said there were "no longer" range restrictions on arms supplied to Ukraine.
"This means that Ukraine can now defend itself, for example, by attacking military positions in Russia... with very few exceptions, it didn't do that until recently. It can now do that," Merz said.
Reuters reported that Zelensky was due to travel to Berlin on Wednesday, although this has not been confirmed.
The BBC approached the Chancellery for comment on whether Merz's statement suggested an announcement was imminent on the supply of Taurus missiles - something that the previous German government refused to do.
Last year, the UK said that Ukraine had the right to decide how to use British supplied weapons in its defence. In November, then-US president Joe Biden gave Ukraine permission to use long-range missiles supplied by the US to strike Russia, albeit with limitations.
The Taurus missile has a range of about 500km - a far greater distance than other systems supplied by Ukraine's allies. Russia said supply of the weapon would be "a dangerous move".
Speaking in New Jersey late on Sunday, Trump said of Putin: "I've known him a long time, always gotten along with him, but he's sending rockets into cities and killing people, and I don't like it at all."
He also said he was considering increasing US sanctions on Russia - something he has repeatedly threatened to do before.
Trump posted his "crazy" remark shortly afterwards, adding on Truth Social: "I've always said that he wants all of Ukraine, not just a piece of it, and maybe that's proving to be right, but if he does, it will lead to the downfall of Russia!"
But the US president also had strong words for Zelensky, saying that he was "doing his country no favours by talking the way he does".
"Everything out of his mouth causes problems, I don't like it, and it better stop," Trump wrote of Zelensky.
Despite Kyiv's European allies preparing further sanctions for Russia, the US has said it will either continue trying to broker these peace talks, or "walk away" if progress does not follow.
Peskov said on Monday that Russia was "truly grateful" to the Americans and "personally to President Trump" for their help in organising and launching this negotiation process.
Last week, Trump and Putin had a two-hour phone call to discuss a US-proposed ceasefire deal to halt the fighting.
The US president said he believed the call had gone "very well", adding that Russia and Ukraine would "immediately start" negotiations toward a ceasefire and "an end to the war".
Ukraine has publicly agreed to a 30-day ceasefire but Putin has only said Russia will work with Ukraine to craft a "memorandum" on a "possible future peace" - a move described by Kyiv and its European allies as delaying tactics.
The first direct Ukrainian-Russian talks since 2022 were held on 16 May in Istanbul, Turkey.
Aside from a major prisoner of war swap last week, there was little or no progress on bringing a pause in fighting closer.
Russia currently controls about 20% of Ukrainian territory. This includes Crimea - Ukraine's southern peninsula annexed by Moscow in 2014.
