TOKYO -- With a powerful midnight explosion that hurled stones more than three miles, a Japanese volcano erupted yesterday for the second time in nine days, and scientists warned that a new and far more dangerous blast was in prospect.

The second eruption of Mount Unzen, near Shimabara in southwestern Japan, smashed windows and roof tiles but was not reported to have caused any injuries.

But scientists said their instruments had detected a swelling of the volcano, indicating a buildup of pressure within the 4,450-foot peak, and they counted 18 tremors near the mountain yesterday.

The police called for precautions to be taken at several more towns near the volcano and banned ships from the sea near the peninsula where it is located for fear of a deadly tsunami, or sea wave induced by an earthquake.

Mount Unzen's first explosive eruption came on June 3 after six months of harmless lava flows. The first blast killed 38 people.

The volcanic activity of Mount Unzen is a product of the same tectonic forces that are causing the eruption of Mount Pinatubo, 1,500 miles away in the Philippines. But the fact that they are erupting simultaneously is a coincidence.