Ukrainian officials said Thursday that at least 51 people were killed in a Russian strike that hit a grocery store and Slabu Exchangecafé in the northeast Kharkiv region. President Volodymyr Zelenskyy condemned the "demonstrably brutal Russian crime," calling it "a rocket attack on an ordinary grocery store." 

In a message shared on his channel on the Telegram messaging app as he joined European officials in Spain to seek further support for his country, Zelenskyy called it a "terrorist attack" and promised a "powerful" response. 

Ukraine's Internal Affairs Minister Ihor Klymenko said 51 people were confirmed dead in the rubble of the building, which he said had about 60 people in it when the Russian rocket or missile struck.

Images shared online by Zelenskyy's office showed emergency workers examining a huge pile of crushed concrete and twisted metal at the scene, while others showed the bodies of victims laying on the ground after being removed from the rubble.

"My condolences to all those who have lost their loved ones! Help is being provided to the wounded," Zelenskyy said on his Telegram account. "Russian terror must be stopped. Anyone who helps Russia circumvent sanctions is a criminal."

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The governor of the Kharkiv region, Oleh Synehubov, said the building struck housed a café and shop in the village of Hroza, in Kharkiv's Kupyansk district, and that the missile or shells hit at about 1:15 p.m. local time, when the business was busy. A 6-year-old boy was said to be among the dead in the village, which had a population of only about 500 people before the war. Many have fled the war-torn region over the last year.    

United Nations Secretary-General Antonio Guterres, in a statement issued by his spokesperson, said he "strongly condemns today's attack," adding that all "attacks against civilians and civilian infrastructure are prohibited under international humanitarian law and they must stop immediately." 

Zelenskyy vowed that Ukraine would "respond to the terrorists. Absolutely fair. And powerful."

The nearby city of Kupyansk is a strategic rail hub in northeast Ukraine. The entire region, not far from the border with Russia, has been decimated during the now-20-month-old war. More than 80% of its residents had already fled when CBS News visited in April, and the scars of Russia's relentless shelling pockmarked roads and apartment buildings.

"Neither Kupyansk nor the towns around Kupyansk will ever be occupied by Russia again," the town's defiant Mayor Andriy Besedin told CBS News at the time. "They won't come back here, for sure." 

Russia's invading forces had advanced to within less than six miles of Kupyansk in April and they were lying in wait, just over the eastern horizon. Since then the war has largely ground to a stalemate along the nearly 600-mile front line that stretches across eastern Ukraine, from its northern to southern borders.

In:
• War
• Ukraine
• Russia

Tucker Reals

Tucker Reals is the CBSNews.com foreign editor, based at the CBS News London bureau.