Sutopo Purwo Nugroho, a spokesman for the National Disaster Mitigation Agency (BNPB), said on Monday that the alert status was raised from level 2 to level 3, which meant the volcano was showing signs of erupting.

He said scientists recorded increasing volcanic activity on Sunday. “There was phreatic eruption [a steam eruption without lava] which happened at the Timbang crater, one of the active craters,” Sutopo said.

He said the crater spouted jets of carbon monoxide and carbon dioxide — both colorless and odorless.

Mount Dieng has six craters: Sileri, Siglagah, Condrodimuko, Sikidang, Sinila and Timbang. Scientists say the latter two are the most active.

Sutopo added that the Volcanology and Geological Disaster Mitigation Agency (PVMBG) had declared a one-kilometer radius around Timbang off-limits. Several villages were covered by the danger zone.

“We have evacuated 1,179 people from Simbar and Sumber villages to [a village in] Batur subdistrict,” Sutopo said.

PVMBG chief Surono said on Monday that Mount Dieng’s eruption was marked by the emission of gas, which if inhaled in large amounts could be fatal.

He said that in 1979, CO2 from the Timbang crater killed 149 people. During the rainy season, the greenhouse gas stays close to the surface of the earth, making it deadlier, Surono added.

Muhammad Djasri, the Banjarnegara district chief, said the government on Monday declared a state of emergency in the area through to June 12.

Sarkono, head of Batur subdistrict, said that although residents in the danger zone had already been evacuated, many villagers were sneaking back to the Mount Dieng area to tend to their land and livestock.