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Japan's Meteorological Agency has raised the alert level for the volcano, and authorities said they were checking if there were any climbers on the mountain at the time of the eruption.
Japanese authorities on Wednesday warned of the threat of lava flows and falling rocks after a volcano erupted on the main southern island of Kyushu.
Mount Aso is one of the world's largest active volcanoes and the largest volcano in Japan. The 1,592-meter (5,223-foot) volcano is a popular tourist destination.
The eruption sent stones tumbling down its grassy slopes and plumes of ash 3.5 kilometers (2.2 miles) high, the Japan Meteorological Agency (JMA) said.
It erupted at about 11:43 a.m. local time (0243 UTC), according to the agency.
JMA on Wednesday raised its warning for Aso to three out of five.
There have been no reports of casualties, but authorities were checking if any hikers had been trapped or injured, officials told local media.
For those near the mountain, "caution must be exercised for large flying rocks and flows of pyroclastic materials," said JMA official Tomoaki Ozaki.
"Caution is warranted even in far-away areas downwind, as the wind may carry not just ash but also pebbles," Ozaki told a televised press conference, warning that toxic gases may also have been emitted.
Television footage showed dozens of vehicles and tour buses parked at a nearby museum that has a clear view of the volcano.
Torrents of ash were seen rushing down Aso's slopes toward the museum, but it did not reach the site.
The last time JMA's warning was raised to Wednesday's level was when Aso erupted in 2016.
Japan is one of the most volcanically active countries in the world.
The island country sits on the so-called "Ring of Fire," where several of the planet's quakes and volcanic eruptions are recorded.
In 2014, Japan saw its deadliest volcanic eruption in almost nine decades on Mount Ontake, in the central Nagano prefecture. The eruption killed 63 people.
fb/sms (AFP, dpa, Reuters)