Ukraine’s Air Alert app, which warns users about air strikes, was hacked on Sunday, the Ukrainian Air Force said. The force said the app sent false alerts about a nuclear or chemical threat and street fighting, while there was no radiation danger, and urged people to remain calm.

A single report said some air sirens also sounded incorrectly, regional governors warned residents not to click links in the false messages, and the system was restored around midnight. The Air Force said the app was probably compromised by “the enemy.”