US President Donald Trump on Sunday said a Russian strike on the Ukrainian city of Sumy that killed at least 34 people was "a horrible thing."

"I think it was terrible. And I was told they made a mistake. But I think it's a horrible thing. I think the whole war is a horrible thing," Trump told reporters on board Air Force One while headed back to Washington.

Asked to clarify what he meant by a "mistake," Trump said that "they made a mistake... you're gonna ask them" — without specifying who or what he meant.

The American leader's National Security Council (NSC) had earlier Sunday called the Russian strike "a clear and stark reminder of why President Donald Trump's efforts to try and end this terrible war comes at a crucial time."

Neither Trump nor the White House named Moscow as the perpetrator of the attack, though Secretary of State Marco Rubio earlier offered condolences to the "victims of today's horrifying Russian missile attack on Sumy."

The Sumy strike came two days after US presidential envoy Steve Witkoff travelled to Russia to meet President Vladimir Putin and push Trump's efforts to end the war. 

Zelensky on Sunday urged the US president to visit his country to better understand the devastation wrought by Russia's invasion.

"Please, before any kind of decisions, any kind of forms of negotiations, come to see people, civilians, warriors, hospitals, churches, children destroyed or dead," the Ukrainian leader said in an interview broadcast on US network CBS.