Huge plumes of ash and smoke were filmed erupting from Mount Etna, Sicily on 27 December as skiers used the slopes below.
Scientists at the Istituto Nazionale di Geofisica e Vulcanologia said volcanic activity at the site, which frequently erupts, had intensified, with craters continuously emitting ash.
In response, scientists issued a red Volcano Observatory Notice for Aviation, its highest level, though authorities said flights would continue to operate normally at a nearby airport unless ashfall increased.