It was the dead of night and a fire was raging in the apartment building where a Russian drone had struck just minutes earlier. Through the smoke, residents stumbled down the stairs from their apartments and told fire officers who were trying to account for all the inhabitants that a young woman was renting the top-floor flat.

Artem, 37, was one of several officers on duty that night, March 13, who raced up to try to find her. On the fifth floor, they broke open the metal door of the woman’s apartment, and dense black smoke billowed into the stairwell. On the other side of the door, they looked into a void.

“There was no apartment,” said Artem, who gave only his first name for security reasons. “There was a meter of floor and then nothing.”

That strike, which killed four people in the building, was one of many that have rained down for months on the northeastern Ukrainian city of Sumy, just 25 miles from the border with Russia, and its surrounding region. Ukrainian officials have warned with increasing urgency that Sumy is a target of a new offensive by Russian forces massing across the border.

“The mood is very anxious,” said Capt. Dmytro Lantushenko, 38, spokesman for the 117th Brigade of the Territorial Defense Forces, based in Sumy. “People read the news, people read Telegram channels, and they cannot ignore the news about a possible attack on Sumy.” Telegram is one of the most widely used social media channels in Ukraine.