The weather agency raised the volcanic alert level Monday for Satsuma-Iojima Island in Kagoshima Prefecture, southwestern Japan, citing an increase in volcanic earthquakes.

The Japan Meteorological Agency raised the alert level for the volcano from 1 to 2 on a scale of 5, warning against approaching the crater. It had previously advised people that the volcano is active.

The agency warned that areas within approximately 1 kilometer of the crater could be affected by flying rock if an eruption occurs.

It observed volcanic temblors on the island 12 times on Thursday and 51 times as of 11:30 a.m. Monday. The number has been increasing since mid-February, it said.

On Friday, the agency also confirmed a type of tremor that indicates underground magma activity.