Smoke rises from Mount Ontake, which straddles Nagano and Gifu prefectures September 27, 2014, in this photo taken and released by Kyodo. The Japanese volcano erupted on Saturday, injuring at least eight people, leaving more than 250 people stranded near the peak and forcing aircraft to divert their routes, officials and media said. [Photo/Agencies]

TOKYO - A Japanese volcano erupted on Saturday, injuring at least eight people, leaving more than 250 people stranded near the peak and forcing aircraft to divert their routes, officials and media said.

A thick, grey cloud of ash rose into the sky above Mount Ontake and a witness said small rocks were being hurled into the air along with the ash. The number of injured was expected to rise, an official said.

"It was like thunder," a woman told broadcaster NHK. "I heard boom, boom, then everything went dark."

The Meteorological Agency said the volcano, which straddles Nagano and Gifu prefectures 200 km (125 miles) west of Tokyo, erupted just before midday and sent ash down the mountain's south slope for more than three km (2 miles). There was no sign of lava from the TV footage.

"Seven people were lightly injured and one person suffered serious injuries as a result of the eruption," Makoto Hasegawa, of the Nagano prefecture fire department, told Reuters.

"Planes are diverting their flying routes to avoid the ash."

NHK quoted police as saying more than 250 people were stranded near the top of the mountain, which last erupted in 2007, and rescue workers were on their way.