World leaders condemned the latest Russian missile strike on Ukraine, one of the deadliest attacks in months, while US President Donald Trump called it a "horrible thing" and a "mistake".

Two ballistic Iskander-M/KN-23 missiles hit the centre of the northeastern city of Sumy, close to the Russian border, on Sunday morning, killing at least 34 people and wounding more than 100 others, Ukrainian authorities said.

The Russian attack damaged 20 buildings in total, including a university, five apartment buildings, cafes, shops and the district court, he added.

The 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 more than three-year war.

"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," US President Trump said on board Air Force One while headed back to Washington on Sunday evening.

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 was referring to.

US Secretary of State Marco Rubio called the attack "horrifying" and a "tragic reminder of why President Trump and his administration are putting so much time and effort into trying to end this war and achieve durable peace".

UN Secretary-General Antonio Guterres was "deeply alarmed and shocked" by the strike, which highlighted a "devastating pattern of similar assaults on Ukrainian cities and towns in recent weeks", his spokesman Stephane Dujarric said.

France's President Emmanuel Macron said the strike showed Russia's "blatant disregard for human lives, international law and the diplomatic efforts of President Trump".

British Prime Minister Keir Starmer said he was "appalled" by the attack, which Italian counterpart Giorgia Meloni described as a "cowardly" act by Russia.

Germany's chancellor-in-waiting Friedrich Merz described it as "a serious war crime, deliberate and intended". (AFP)