Flights from Catania airport in Sicily were grounded on Sunday after volcanic ash from an eruption of Mount Etna covered the runways.
Pictures and video footage show black volcanic ash covering planes at the airport, and blanketing roads in the area.
The observatory said in a statement that the eruptive phase ended around 9 p.m. Sunday, but it couldn’t predict when a new phase might begin.
Volcanic ash clouds are a serious hazard to aviation, reducing visibility, damaging flight controls and ultimately causing jet engines to fail.
In 2010, a volcanic eruption in Iceland sent a huge plume of ash moving across the Atlantic, causing massive disruption to air traffic across Western Europe.
In the aftermath of the crisis, international aviation regulators introduced new risk management guidance on flight safety and volcanic ash.
Mount Etna is Europe’s most active volcano, and also the highest, at about 3,350 meters (almost 11,000 feet) tall.
It is located on the east coast of Sicily, just north of the city of Catania.
Etna’s eruptions are relatively frequent, resulting in a landscape that constantly changes as lava hardens, craters collapse and, in some cases, are created.
It is one of Sicily’s most popular tourist attractions, with visitors able to hike up the volcano or take a cable car that connects with a 4×4 bus to take visitors to the 2,800-meter mark.
In March 2017, a group of tourists and a BBC camera crew made a dramatic escape during an eruption.
Surrounded by steam and with boiling hot rocks pelting down on them, the group sprinted to a nearby rescue vehicle that took them to safety. Ten people were injured, but none seriously, Italian news agency Ansa reported at the time.