The government expanded the no-go zone at Mount Shinmoe in Kyushu on Sunday as volcanic activity intensified.
The Meteorological Agency issued a warning to stay at least 3 km (about 2 miles) away from the crater of the 1,421-meter-high volcano straddling Kagoshima and Miyazaki prefectures which erupted Wednesday and continues to spew ash.
The latest warning follows a significant increase in volcanic gas emitted, the agency said, warning of a possible escalation of eruptions.
Mount Shinmoe last erupted in 2011.