By Kit Maher and Rob Picheta, CNN

(CNN) — President Donald Trump said Sunday that Vladimir Putin “has gone absolutely CRAZY,” after the Russian leader launched the largest aerial attack of Moscow’s three-year full-scale war on Ukraine.

“I’ve always had a very good relationship with Vladimir Putin of Russia, but something has happened to him. He has gone absolutely CRAZY! He is needlessly killing a lot of people, and I’m not just talking about soldiers. Missiles and drones are being shot into Cities in Ukraine, for no reason whatsoever,” Trump posted on Truth Social.

Hours earlier, Trump had told reporters: “I’m not happy with what Putin is doing. He’s killing a lot of people, and I don’t know what the hell happened to Putin.”

Moscow launched overnight attacks on Friday, Saturday and Sunday in Ukraine, killing at least 29 people over three days and injuring dozens more, according to officials. The bombardment from Saturday into Sunday saw more drones and missiles launched at Ukraine than in any other single assault in three years.

Trump’s intervention, coming at a crucial moment in Moscow’s war, will be viewed with relief in the West as a signal that the White House has grown frustrated with Putin’s refusal to accept a proposed 30-day ceasefire in the conflict.

The Kremlin on Monday sought to downplay the significance of the remarks, categorizing them as an “emotional reaction.”

“This is a very important moment, which is associated with the emotional overload of absolutely everyone and with emotional reactions,” Kremlin spokesperson Dmitry Peskov said Monday.

He also said Moscow is “truly grateful to the Americans and personally to President Trump” for his efforts to bring Moscow and Kyiv to the negotiating table.

But the clash could have consequences for Russia. Trump also told reporters Sunday evening that he’s “absolutely” considering additional sanctions against Russia, which Kyiv has pushed for. Trump previously said he would not join in any new sanctions because he thought “there’s a chance” of progress, but remarked that could change.

But Trump also turned his criticism toward Ukrainian President Volodymyr Zelensky, who said Sunday morning that “America’s silence” encourages Putin to continue his onslaught.

“Likewise, President Zelenskyy is doing his Country no favors by talking the way he does,” Trump wrote. “Everything out of his mouth causes problems, I don’t like it, and it better stop.”

At least 25 people were killed in two rounds of overnight attacks in Ukraine over the weekend, including children, officials said, and dozens more people were injured. At least four people were killed overnight into Monday, officials added.

Russia has been intensifying its aerial bombardment of Ukraine as international pressure mounts on Putin to agree to the ceasefire plan, which has the backing of the United States and Kyiv.

“The world may go on vacation, but the war continues, despite weekends and weekdays. This cannot be ignored. America’s silence, and the silence of others in the world, only encourages Putin,” Zelensky said Sunday morning.

Russia’s attacks set up another critical week in discussions over an end to the conflict. Earlier this month, teams from Kyiv and Moscow met face-to-face for the first time since the war’s early weeks, but the flicker of hope that surrounded those talks was seemingly extinguished when Moscow pressed ahead with its assaults on Ukrainian cities and again rejected calls to agree to a 30-day halt in fighting.

Some progress was made; both countries agreed to a massive prisoner swap, which began on Friday and will see 1,000 inmates released by each side.

But attention quickly returned to the fighting, which has intensified in recent weeks. Moscow last month claimed to have finally retaken the Kursk region, which Kyiv captured large sections of last summer, taking a significant piece of leverage back from Ukraine.

“We must be prepared for this war to last longer than we can all wish or imagine,” German Chancellor Friedrich Merz told public broadcaster ARD on Monday. “The question is whether the USA will continue to see it as its political and military duty to help Ukraine, and that is what we are trying to achieve.”

“Trump (is) obviously increasingly disillusioned with Putin from the massive attacks on Kyiv and other cities in Ukraine over the weekend. In this respect, I hope that America stays on board,” Merz said.

Trump said Sunday he was “very surprised” about the latest Russian aerial assault, though just a week ago, Russia launched its largest drone attack against Ukraine – a day before Putin and Trump spoke on the phone. “We’re in the middle of talking, and he’s shooting rockets into Kyiv and other cities,” Trump told reporters in New Jersey on his way back to Washington.

Trump, who has often touted his good relationship with Putin, spoke with the Russian leader on Monday in an attempt to negotiate a 30-day ceasefire between Russia and Ukraine.

After the call with Putin, and subsequent calls with Zelensky and other European allies, Trump announced that “Russia and Ukraine will immediately start negotiations toward a Ceasefire and, more importantly, an END to the War.”

Trump added that conditions “will be negotiated between the two parties, as it can only be, because they know details of a negotiation that nobody else would be aware of.”

This story and headline have been updated with additional developments.

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CNN’s Anna Chernova contributed reporting