An eruption was observed Wednesday at Mt. Aso in southwestern Japan, but 16 climbers on the volcano at the time walked away uninjured, authorities said Tuesday.

The eruption occurred at 11:43 a.m. at the No. 1 Nakadake crater, according to the Japan Meteorological Agency. Mt. Aso's volcanic alert level was raised to 3 on a scale of 5, with entry restricted to areas within 2 kilometers of the crater.

Supplied photo captured by a Japan Meteorological Agency camera shows Mt. Aso erupting in Kumamoto Prefecture on Oct. 20, 2021. (Kyodo)

Volcanic ash blew more than 1 kilometer from the crater and reached a height of about 3,500 meters, while a pyroclastic flow was confirmed within 1.3 km west of the crater for the first time since October 2016, according to the agency.

Large volcanic cinders were also ejected to around 900 meters south of the crater, it said.

At an extraordinary press conference Wednesday, the agency said there were no signs warranting the need to raise the alert level to 3 prior to the eruption.

The latest eruption came after a small-scale one on Thursday. The agency had raised the alert level for Mt. Aso from 1 to 2 the day prior.

Photo taken from a Kyodo News helicopter shows volcanic fumes billowing from Mt. Aso as it erupts explosively in Kumamoto Prefecture, southwestern Japan, on Oct. 20, 2021. (Kyodo) ==Kyodo