US President Donald Trump has said he is "not happy" with his Russian counterpart Vladimir Putin, following Moscow's largest aerial attack yet on Ukraine.

In a rare rebuke, Trump said: "What the hell happened to him? He's killing a lot of people." He later called Putin "absolutely crazy".

Ukrainian President Volodymyr Zelensky earlier said Washington's "silence" over recent Russian attacks was encouraging Putin, urging "strong pressure" - including tougher sanctions - on Moscow.

At least 12 people were killed and dozens injured in Ukraine overnight Sunday after Russia fired 367 drones and missiles - the highest number in a single night since Putin launched a full-scale invasion in 2022.

Air sirens warning of incoming drones and missiles sounded again in many regions of Ukraine early on Monday.

At least three people, including a child, were injured in the north-eastern city of Kharkiv, Mayor Ihor Terekhov said.

Speaking to reporters 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."

Asked about whether he was considering increasing US sanctions on Russia, Trump replied: "Absolutely." The US president has repeatedly threatened to do this before - but is yet to implement any restrictions against Moscow.

Shortly afterwards, Trump wrote in a post on Truth Social that Putin "has gone absolutely crazy".

"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 "is 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.

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.

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 pausing in fighting closer.

Russia currently controls about 20% of Ukrainian territory. This includes Crimea - Ukraine's southern peninsula annexed by Moscow in 2014.