President Volodymyr Zelenskiy demanded a tough international response against Moscow over Sunday's attack, which came with US President Donald Trump's push to rapidly end the war struggling to make a breakthrough.
Moscow said on Sunday it had hit a meeting of Ukrainian officers with "Western colleagues" in Sumy, but did not identify any Western participants or provide evidence to support the allegation.
The leaders of Britain, Germany and Italy condemned the attack. Trump, when asked about the Russian strike, said that it was terrible.
"And I was told they made a mistake," he said without elaborating further.
"But I think it's a horrible thing."
Dead bodies were strewn on the ground in the middle of a city street near a destroyed bus and burnt-out cars in a video Zelenskiy posted on social media.
"Only scoundrels can act like this, taking the lives of ordinary people," Zelenskiy said, noting that the attack had come on Palm Sunday when some people were going to church.
Zelenskiy, in an interview with CBS News' 60 Minutes aired on Sunday, urged Trump to visit Ukraine.
"Please come to see people, civilians, warriors, hospitals, churches, children, destroyed or dead," Zelenskiy said in a video clip the program posted on social media.
During the interview, which took place on Friday, Zelenskiy was asked if the United States had Ukraine's back.
After a brief pause, Zelenskiy replied: "Even in this pause of mine, there's a problem, because I want to answer truthfully and quickly that the United States is our strategic, strong partner," he said.
"But the pause is doubt. I don't doubt that the people of America are with us, but in a long war, many details are forgotten."
He called on the United States to provide forces as part of an international peacekeeping effort, specifically asking for Washington to help protect Ukrainian airspace with aircraft.
Under Trump's administration, US officials have held separate rounds of talks with Kremlin and Kyiv officials to try to move towards a cessation of hostilities in Ukraine.
A separate Russian drone attack injured five people in the Black Sea port city of Odesa late on Sunday and damaged a medical facility, regional officials said.
The Sunday attacks followed a missile strike on the central Ukrainian city of Kryvyi Rih, Zelenskiy's hometown and far from the ground war's front lines in the east and south, this month that killed 20 people, including nine children.
Sumy, with a population of around a quarter of a million and located just over 25 km from the Russian border, became a garrison city when Kyiv's forces launched an incursion into Russia in August.
Sumy's acting mayor, Artem Kobzar, announced three days of mourning for the victims starting from Monday.
The people who were caught in Sunday's strike were out on the street or inside cars, public transport and buildings when the missiles hit, Interior Minister Ihor Klymenko said.
"Deliberate destruction of civilians on an important church feast day," he wrote.
Andriy Yermak, Zelenskiy's chief of staff, said the missiles contained cluster munitions. "The Russians are doing this to kill as many civilians as possible," he said.
Russia currently holds nearly 20 per cent of Ukraine's territory in the east and south, and its forces have been slowly advancing in the east.
Ukraine's Foreign Minister Andrii Sybiha said Kyiv was "sharing detailed information about this war crime with all of our partners and international institutions."
The International Criminal Court in The Hague, which Ukraine officially joined this year, is conducting investigations into high-profile cases of alleged war crimes in the conflict.
