The state vulcanology office placed the mountain on the second-highest of four alert levels.
A noxious mixture of carbon dioxide and toxic substances seven times normal levels has been recorded from the volcano in recent days. Seeping hot gasses have caused a lake near the crater to bubble as if it were boiling.
Meanwhile, officials cancelled all leaves for military and emergency personnel to ensure an adequate evacuation response to a violent eruption. Officials said that evacuation drills had been conducted in at least nine villages near Mount Kelud in the past week.
Laborers have been dispatched to strengthen and expand dams on the east of Java island to be able to hold back deadly mud flows if the restive volcano should erupt.
The vulcanology office warned that signs of an imminent explosive eruption were much stronger now than those that preceeded Kelud’s last major eruption in 1990.
A 1919 eruption sent boiling water cascading down its slopes, killing 5,000 people in 104 villages.