Unknown hackers have targeted Ukraine’s nationwide emergency alert mobile app, sending residents in Kyiv a warning that they were at risk of an imminent nuclear strike.

The app reported a high possibility of nuclear radiation late on Sunday. It also wrongly warned of a chemical threat to the Ukrainian capital and street-to-street fighting. The messages were written in English with spelling mistakes, as well as in Ukrainian.

Many residents ignored the alerts, which began shortly before 11pm local time, assuming that the app had been compromised. Others were scared. “I was briefly terrified,” one woman, Nina, said. “I lay on my bed and wondered what was coming.”

A message written in Russian also claimed Ukraine was about to be hit by Oreshnik missiles, which can carry nuclear warheads. The Kremlin has used the ballistic weapon three times before, hitting the Kyiv region, and the cities of Dnipro and Lviv.

In a post on Sunday, the Ukrainian emergency app said “unauthorised parties” had interfered with its operation. The channel had been temporarily suspended, it said, warning users not to click on links in “suspicious notifications”.

It was unclear on Monday who might have carried out the cyber-raid, or why. Russian officials claimed hackers had also sent fake emergency alerts there after gaining access to the app of the ministry for emergencies, based in Moscow.

One of their posts mentioned the hacking group Hitcore. Mash, a Russian Telegram channel with links to the security services, said it had spoken to representatives of Hitcore and that they had claimed responsibility for the attack. The report could not be verified.

The fake nuclear threats to Ukraine came as Russia resumed its attacks on the southern city of Kherson, just hours after the end of a ceasefire during which hundreds of people were evacuated from the nearby Kremlin-controlled town of Oleshky.

Kherson’s military administration said a Russian drone hit a hospital at 7.40am. Video footage showed wrecked rooms, with windows blown out, doors broken, and debris on the street. Nobody was hurt, officials said.

There is evidence that the Russian army used the six-day ceasefire, which the White House said was brokered by the first lady, Melania Trump, to strengthen its positions in Oleskhy and to rotate in new troops. Bodies of dead civilians were also removed from a hospital morgue.

Footage shared with Sky News showed Russian soldiers deploying heavy military equipment and delivering ammunition. They carried large bags into burned out houses and buildings and disappeared into basements, the broadcaster reported.

Armoured vehicles were spotted moving around Oleshky’s streets. The occupied city is 3 miles from Kherson, on the opposite bank of the Dnipro River. Before last week’s truce, resupply was difficult because of the threat from Ukrainian kamikaze drones.

According to the commander of Ukraine’s 34th Separate Marine Brigade, Dmitry Pulinets, Russian soldiers were previously unable to drive in the city, “neither during the day, nor at night, nor in bad weather”.

“Now they have this opportunity. And for the last few days this movement has been happening constantly,” he told Sky News.

The ceasefire expired at 11pm on Sunday. Both sides hailed the local pause in the fighting – the most significant since 2022 – as a success. Ukraine’s ombudsman, Dmytro Lubinets, who visited Oleshky to coordinate the mission, said it could be repeated on other parts of the frontline.

However, Vladimir Putin’s spokesperson Dmitry Peskov said on Monday there were no plans for another truce elsewhere. He said it was “standard practice” for army units to regroup in “such circumstances” and added that he assumed Ukraine’s armed forces had “done the same”.