A small volcanic eruption at a Japanese hot springs resort not far from Tokyo prompted authorities on Tuesday to further limit access to the area, warn that more eruptions were possible and urge a handful of people to evacuate.

Japan, one of the world's most seismically active nations, has suffered a recent spate of eruptions, including one that forced the evacuation of a southern island. In September, 63 people died when a peak crowded with hikers suddenly erupted.

Volcanic ash was spat from a valley on Mount Hakone, which has been belching out unusual amounts of steam in recent months, forcing officials to close part of the resort at the start of the spring tourist season.

There were no reports of injury or damage, and roughly 40 people were urged to evacuate.

Japan's Meteorological Agency raised the warning level on the mountain to 3 from 2, closing a broader area, and an agency official said activity in the area, some 80 km (50 miles) west of Tokyo, seemed to have risen "to a new level".

"It was an extremely small scale eruption, but there is the chance of a larger one that could affect a wider area," he told a news conference.

Hakone is a resort famed for its hot springs and views of Mount Fuji. More than 21 million people visited in 2014, including 217,000 from overseas, the Hakone town office said on its website.

White clouds, apparently steam, billowed up from vents in the stark Owakudani, "Great Boiling Valley", the 1,044-metre (3,425 ft) high area around a crater created during an eruption of Mount Hakone 3,000 years ago.

Predicting the scale of any eruption is hard because the mountain last erupted 800 years ago, said volcanologist Toshitsugu Fujii, an emeritus professor of Tokyo University.

"If hot water or magma becomes involved, it could explode at a deeper level, and there would probably be very little warning," he said.

"Things are now taking place at a shallow level and probably it won't go that far. But you can't say when that might change."

Fujii did say it was highly unlikely that Hakone's activity foretold an eruption of iconic Mount Fuji, which used to erupt every 30 years but has been silent since 1707.

A catastrophic eruption of the 3,776-metre-high (12,390-ft-) peak could rain some 10 cm of ash on Tokyo, located 100 km to the northeast.