Mount Aso erupted Wednesday, spewing a giant column of ash thousands of meters into the sky as hikers rushed away from the popular tourist spot.
No injuries were immediately reported after the late-morning eruption in Kumamoto Prefecture, which sent rocks flying in a dramatic blast captured by nearby CCTV cameras.
People were warned not to approach the volcano as it ejected hot gas and ash as high as 3,500 meters, and sent stones tumbling down its grassy slopes.
Authorities said they were checking if any hikers had been trapped or injured, as TV footage showed dozens of vehicles and tour buses parked at a nearby museum that has a clear view of the volcano.
Pale gray torrents of ash were seen rushing down Aso's slopes toward the museum, but did not reach the site.
"Human lives are our priority and we are working with the Self-Defense Forces, police and firefighters to effectively deal with the situation," Chief Cabinet Secretary Hirokazu Matsuno told reporters.
For those near the mountain, "caution must be exercised for large flying rocks and flows of pyroclastic materials," said Meteorological Agency official Tomoaki Ozaki.
"Caution is warranted even in far-away areas downwind, as the wind may carry not just ash but also pebbles," Ozaki said at a televised news conference, warning that toxic gases may also have been emitted.
The last time the agency raised its warning for Aso to Wednesday's level — three out of five — was when it erupted in 2016.
The agency has been warning of increasing volcanic activity there in recent days, including a small eruption on Thursday.
Aso's huge caldera dominates the southwestern main island of Kyushu, where the 1,592-meter volcano is a popular tourist draw.
Japan is one of the world's most volcanically active countries.
It sits on the so-called Pacific Ring of Fire, where a large proportion of the planet's quakes and volcanic eruptions are recorded.
In September 2014, Japan suffered its deadliest eruption in almost 90 years when Mount Ontake, in Nagano Prefecture, burst unexpectedly to life, killing an estimated 63 people.
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