No casualties or injuries were immediately reported after the eruption but Gunawan warned nearby residents not to travel within eight kilometres (five miles) of the crater after the threat level was raised to four.
"A lot of people have started to go down," Thoriqul Haq, the local administration chief for Lumajang, where the volcano is located, told broadcaster Kompas TV.
Mount Semeru last erupted exactly one year ago, killing at least 51 people.
The disaster left entire streets filled with mud and ash, swallowing homes and vehicles, with nearly 10,000 people seeking refuge.
Indonesia sits on the Pacific Ring of Fire, where the meeting of continental plates causes high volcanic and seismic activity.
The Southeast Asian archipelago nation has nearly 130 active volcanoes.
