Efforts to estimate the size of the dome, already visible in the crater lake, were hampered by thick clouds of steam.
The area around Mount Kelud, among the deadliest of Indonesia's 100 active volcanoes, has been on the highest level of alert for more than two weeks.
Surono, one of 16 volcanologists monitoring activity at Mount Kelud around the clock, said the temperature in the lake was so great that their equipment nearby had stopped working.
Some 350,000 people live in the area around Mount Kelud and several thousand have been moved to government shelters.
However, tens of thousands more have ignored evacuation warnings, choosing to stay behind to tend their crops and livestock.
In 1919, Mount Kelud spewed scalding water from its crater lake, killing about 5,000 people.
|Hundreds fled their villages as Mount Kelud|
entered a critical phase [EPA]
Scientists predict that Mount Kelud's eruption could be bigger than the one in 1990 which killed at least 30 people.
But the monitoring team has also said that given the unpredictable nature of the 1,731-metre volcano a smaller or gradual eruption was possible, or it was possible that there would be no eruption at all.
Kelud, also known as Kelut, means "sweeper" in Javanese, a reference to the fact that when it erupts, it sweeps away everything in its path.
Indonesia is prone to volcanic eruptions and earthquakes due to its location within a series of fault lines stretching from the Western Hemisphere through Japan and Southeast Asia, called the "Ring of Fire".
Officials were also concerned about increased activity at two other nearby volcanoes which were spewing hot ash, molten rock and clouds of dark smoke.
A few hundred kilometres away, Anak Krakatao, or the "Child of Krakatoa", fired pumice and lava onto its slopes off the northern tip of Java island.
Anak Krakatao was formed after a massive eruption at the giant Krakatao volcano in 1883.
That blast was heard nearly 3,200 kilometres away in Australia and sent surges of gas and burning ash which, combined with a tsunami, killed at least 36,000 people.
One other volcano, Mount Semeru, about 70 kilometres southeast of Mount Kelud, has sent bursts of ash showering down on nearby villages, coating buildings in and around the town of Blitar with a fine layer of ash.
No evacuations were ordered in the town.