Ash from Semeru traveled up to four kilometers away after the Saturday eruption, Indonesia's geological agency reported.
A sand mine company's office in Kampung Renteng village was buried after the eruption, trapping 15 people, according to foreman Hasim, 65.
“There's no news from them. Only one operator was rescued, he's now at the hospital with burns," he told AFP.
Hasim said he ran home after the eruption.
"It was pitch dark," he added. "It was only 3 pm but it looked like night."
Rescue officials said some were buried inside their vehicles, with no time to escape.
Those who managed to find shelter recounted the horror after the eruption.
Suwarti Ningsi and her daughter were trapped for five hours at home after the eruption.
"I couldn't see anything; it was just like at night. Everybody was panicking," said the 42-year-old.
"I could only pray... for me and my daughter to be saved."
The Red Cross said it had rushed ambulances, medical teams, more than 65,000 surgical masks -- to protect against ash and Covid -- and other emergency supplies to the affected areas.
Threat of rain
The ash and mud have also polluted the waterways around Mt Semeru, turning them into streams of dark grey sludge.
Rain is forecast for the area, which could further hinder rescue work.
There is also a risk of the rain causing ash sediment to form a new river of hot lava, the country's top volcanologist Surono told a local TV station.
Semeru's last major eruption was in December 2020, which also forced thousands to flee and wrecked villages.
Indonesia sits on the Pacific Ring of Fire, where the meeting of continental plates causes high volcanic and seismic activity, and the country has nearly 130 active volcanoes.
In late 2018, an eruption in the strait between Java and Sumatra islands caused an underwater landslide and tsunami that killed more than 400 people.
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