Russia’s weekend bombing of Ukraine has stirred a hornet’s nest in the United States with President Donald Trump saying “I’m not happy with Putin”.

"I don't know what's wrong with him. What the hell happened to him? Right? He's killing a lot of people. I'm not happy about that,” Trump told reporters at the airport in Morristown, New Jersey, as he prepared to return to Washington.

Trump spoke in reaction to a Russian barrage of 367 drones and missiles at Ukrainian cities overnight on Sunday, including the capital Kyiv, in the largest aerial attack of the war so far, killing at least 12 people and injuring dozens more.

Trump, who has generally aligned himself with Russia during the conflict, said he was “surprised” by the escalation, once again minimising the fact that Russia launched the war with an unprovoked invasion.

"Always gotten along with him, but he's sending rockets into cities and killing people, and I don't like it at all," Trump said.

When asked if he was considering increasing US sanctions on Russia, Trump responded, "Absolutely." Although the US President has made similar threats in the past, he has yet to follow through with any actual measures against Moscow.

Shortly after, Trump posted on Truth Social, claiming that Putin "has gone absolutely crazy."

“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. 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!,” Trump wrote.

However, the US President also criticised Ukrainian President Volodymyr Zelensky. “Likewise, President Zelenskyy is doing his Country no favors by talking the way he does. Everything out of his mouth causes problems, I don’t like it, and it better stop.”

Trump also wrote, “This is a War that would never have started if I were President. This is Zelenskyy’s, Putin’s, and Biden’s War, not “Trump’s,” I am only helping to put out the big and ugly fires, that have been started through Gross Incompetence and Hatred.”

Trump has been trying to get both sides to agree to a ceasefire in the three-year-old war and he spoke for more than two hours with Putin last week.