In November, Mount Lewotobi Laki-Laki on the tourist island of Flores erupted multiple times, killing nine people, cancelling scores of international flights to Bali and forcing the evacuation of thousands.

The latest emergency measures around the 1,703-metre (5,587-foot) twin-peaked volcano came after increased seismic and volcanic activity.

"The results of visual monitoring and seismic activity showed... a sudden increase in volcanic earthquakes," Indonesia geological agency head Muhammad Wafid said in a statement Thursday.

"An eruption could potentially occur in the future."

The heightened activity led to the alert level being raised to the highest of a four-tiered system starting from 3:00 am (1900 GMT Wednesday), Wafid said.

He also warned of the possibility of hazardous lahar floods -- a type of mud or debris flow.

"People around Mount Lewotobi Laki-Laki should be on alert for the potential of lahar floods in rivers that originate at Mount Lewotobi Laki-Laki's peak if high-intensity rain occurs," he said.

Wafid later told a press conference that an exclusion zone had been widened from five to six kilometres (three miles) around the crater.

"This includes six villages that must be evacuated," he said, without specifying the number of residents there.

Laki-Laki, which means "man" in Indonesian, is twinned with a calmer volcano named after the Indonesian word for "woman".

Indonesia, a vast archipelago nation, experiences frequent seismic and volcanic activity due to its position on the Pacific "Ring of Fire".