A phreatic explosion is likely to have caused an eruption on Mount Komagatake early Saturday, the first eruption from the mountain in southern Hokkaido in a month, the Sapporo District Meteorological Observatory said.

However, observatory officials said there are no signs that magma has escaped from the mountain's crater, citing an analysis of volcanic ash which fell on the nearby town of Shikabe.

Hokkaido University professor Tadahide Ui, who conducted an aerial survey of the mountain, said Saturday's eruption was slightly bigger than one on Sept. 4.

"While the eruption will probably come to an end, the fact that there have been five small eruptions since 1996 is somewhat akin to the situation that preceded the big eruption in 1929," he said.

The 1929 eruption is believed to have been one of the biggest volcanic eruptions in Japan this century.

Volcanic tremors on the 1,131-meter mountain started at around 2:43 a.m. and the towns of Shikabe and Minamikayabe on the eastern flanks of the mountain were coated in some 1 mm of ash, the observatory said, adding the number of tremors had totaled seven by 1 p.m.

Smoke was rising about 1,200 meters from the crater at around 3 a.m. before subsiding later to between 200 and 1,000 meters, it said.

Five towns below the mountain set up disaster prevention task forces immediately after the observatory's 3:28 a.m. volcanic eruption bulletin. The headquarters were canceled later in the day as the eruption has subsided. Twelve Shikabe residents were temporarily evacuated to the town's gym.

On Sept. 4 and 28, small-scale phreatic explosions occurred on the mountain.