KYIV, Ukraine (AP) — Residents of Ukraine and Russia received phony emergency alerts from government-run mobile apps that were hacked, authorities in both countries said. The phony alerts in Ukraine included a warning about an imminent nuclear threat.

It was not clear if the attacks late Sunday in Ukraine and early Monday in Russia were related. Ukraine said its mobile app, which residents rely on to seek safety from Russian missile and drone strikes, was up and running again after being temporarily suspended. Russian authorities said efforts to restore its emergency alert mobile app were ongoing.

The cyberattacks came as a limited ceasefire expired Monday in Ukraine's southern Kherson region, where an agreement between Ukraine and Russia allowed humanitarian aid and evacuation missions into front-line areas in Russian-occupied territory.

Ukrainian authorities reported multiple Russian drone attacks in the region early Monday, hours after the ceasefire ended.

After cyberattacks, false alerts sent in Ukraine and Russia

A Russian Telegram channel with links to the country's security services said Monday that a hacking group, Hitcore, claimed responsibility for the breach of the Ukrainian emergency alert system. The Associated Press did not independently verify the claim, and the identities, affiliations or motivations of the purported hackers remained unclear.

The alerts received by Ukrainians — making false claims of high radiation levels, a chemical threat and public disturbances in Kyiv — were written in English, Russian and Ukrainian. The English language alerts contained spelling and grammatical errors.

One of the messages, written in Russian, said: “Hi! There will be a nuclear threat across Ukraine. Do not worry, the Oreshnik is incoming,” a reference to a Russian ballistic missile.

There was little indication the fake alerts caused panic in Ukraine. Instead, there was a flood of humorous posts online, with social media users sharing photos of their pets relaxing and apocalypse-themed images generated by artificial intelligence. A photo shared of Kyiv Mayor Vitali Klitschko, a former international kickboxing champion, included the message: “It’s a good thing we have a boxer for a mayor.”

In a Telegram post late Sunday following the breach, Ukraine’s air raid alert channel said “unauthorized parties” had interfered with the app’s operation and warned users not to click on links in “suspicious notifications.”

Ukraine's state-run Center for Countering Disinformation urged people to monitor other emergency channels on social media while the cyberattack was being addressed.

A spokesman for Ukraine’s air force command, Yurii Ihnat, said in a television interview that the cyberattack had no impact on Ukraine’s security and defense operations.

In a statement, Russia’s Emergency Situations Ministry said hackers sent a false alert through its app at approximately 2:45 a.m. Moscow time on Monday across 33 of Russia’s 89 regions. It did not say who it believed carried out the attack.

The ministry said that the digital vulnerability exploited by the attackers had been identified and that “necessary measures” were being taken to prevent a similar breach. It also reassured Russians that the wider public alert system was working normally through SMS messages, media broadcasts and municipal sirens and loudspeakers.

Kremlin spokesperson Dmitry Peskov said all of Russia’s state agencies were under the constant threat of cyberattacks, including a growing number that use artificial intelligence.

“All government bodies must take steps to improve the cybersecurity of our digital resources,” Peskov said.

Limited ceasefire ends in Kherson region

As the ceasefire in Ukraine's Kherson region came to an end on Monday, the regional administration said that a 62-year-old woman and a 73-year-old man were hospitalized with blast injuries in the city of Kherson.

The brief pause in fighting, which began on Oct. 27, was part of a humanitarian aid and evacuation mission in the Russian-occupied city of Oleshky, and marked the most significant truce between Kyiv and Moscow since Russia’s full-scale invasion began nearly five years ago.

Speaking to reporters in Kyiv on Monday, Ukraine’s ombudsman and lead civilian negotiator for the mission, Dmytro Lubinets, said he hoped the localized ceasefire and evacuation would act as a “model” for other such efforts. He said it could “add to the momentum” of three-way negotiations between Ukrainian, Russian and U.S. officials expected this month.

Ukraine’s general staff said Monday that Russia’s military had bolstered its drone defenses and replenished ammunition supplies in the area during the ceasefire.

Elsewhere in Ukraine, emergency services reported that drone strikes injured 12 people in the eastern Kharkiv region and damaged a refrigeration facility, greenhouses and residential buildings.

Five people were also reported injured in the Black Sea port city of Odesa, where a warehouse was targeted and homes were damaged.

Russia’s Ministry of Defense said Monday that it shot down 212 Ukrainian drones overnight, while the head of Russia’s Samara region, Gov. Vyacheslav Fedorishchev, said in a statement that local air defenses were repelling an aerial attack, but did not give further details.

Independent Russian media outlets later reported that Ukrainian drones had targeted one of Europe’s largest oil transport hubs in Samara.

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Katie Marie Davies contributed to this report from Manchester, England.

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Follow the AP’s coverage of the war in Ukraine at https://apnews.com/hub/russia-ukraine