"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 on Sunday, as he prepared to return to Washington.
Trump spoke after 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 has been trying to get both sides to agree to a ceasefire in the three-year-old war in Ukraine and he spoke for more than two hours with Putin last week.
He raised the possibility of imposing more sanctions on Russia in response to the ongoing attacks.
"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.
Ukrainian President Volodymyr Zelenskiy has called on the US, which has taken a softer public line on Russia and Putin, since Donald Trump took office, to speak out.
"The silence of America, the silence of others in the world only encourages Putin," he wrote on Telegram.
"Every such terrorist Russian strike is reason enough for new sanctions against Russia."
It was the largest attack of the war in terms of weapons fired, although other strikes have killed more people.
Interior Minister Ihor Klymenko said 12 people had been killed and 60 more wounded. Earlier death tolls given separately by regional authorities and rescuers had put the number of dead at 13.
"This was a combined, ruthless strike aimed at civilians. The enemy once again showed that its goal is fear and death," he wrote on Telegram.
The assault comes as Ukraine and Russia prepared to conduct the third and final day of a prisoner swap in which both sides will exchange a total of 1000 people each.
US Special Envoy to Ukraine Keith Kellogg said on Sunday the attack was "a clear violation" of the 1977 Geneva Peace Protocols and called for an immediate ceasefire.
Ukraine and its European allies have sought to push Moscow into signing a 30-day ceasefire as a first step to negotiating an end to the three-year war.
Their efforts suffered a blow earlier this week when Trump declined to place further sanctions on Moscow for not agreeing to an immediate pause in fighting, as Kyiv had wanted.
Ukraine's air force said Russia had launched 298 drones and 69 missiles in its overnight assault, although it said it was able to down 266 drones and 45 missiles.
Damage extended to a string of regional centres, including Ukraine's second-largest city, Kharkiv, as well as Mykolaiv in the south and Ternopil in the west. In Kyiv, Tymur Tkachenko, head of the city's military administration, said 11 people were injured in drone strikes.
No deaths were reported in the capital, although four were killed in the region around the city, according to officials.
This was the second large aerial attack in two days.
On Friday evening, Russia launched dozens of drones and ballistic missiles at Kyiv in waves that continued through the night. In northeastern Ukraine, Kharkiv Mayor Ihor Terekhov said early on Sunday that drones hit three city districts and injured three people.
18°C