More than 30 people have been found on Sunday in a state of "cardiopulmonary" arrest near the peak of the Japanese volcano that erupted on Saturday, NHK national news said, without specifying they have died.

Japan does not confirm deaths until a formal examination has been made. A Nagano prefectural official was unable to confirm the NHK report.

Hundreds of people, including children, were stranded on the peak after it erupted without warning, sending ash pouring down the slope for more than 3 km (2 miles.) Most made their way down that evening.

Volcanoes erupt periodically in Japan, one of the world's most seismically active nations, but there have been no fatalities since 1991, when 43 people died in a pyroclastic flow, a superheated current of gas and rock, at Mount Unzen in southwestern Japan.