The volcano erupted at least three times Tuesday, forcing thousands of nearby residents to flee.
Mount Merapi, which looms on the horizon north of the major city of Yogyakarta, is one of Indonesia's most active volcanoes and lies in one of the world's most densely populated areas. The volcano has a summit elevation of nearly 10,000 feet [3,000 meters].
Experts told the BBC that the ash levels had subsided a little, but that their readings suggested there would be more volcanic activity soon.
Thousands fled their homes on Tuesday as ash spewed out of the volcano, turning the landscape white.
But many people refused to leave, and rescuers fear the death toll may rise.
One rescuer, Christian Awuy, told the BBC that he feared up to 50 could have been killed.
He said although 10,000 people had been evacuated, many had stayed behind.
The AFP news agency reported local officials as saying the man known as the volcano's spiritual gatekeeper, Mbah or grandfather Marijan, was among the dead. For many Javanese, Merapi is a sacred site.
The agency said he was found dead in his house about 4km (2.5 miles) from the summit, but this has not been independently confirmed.
A cameraman for Reuters was quoted as saying: "Several houses and cattle have been burned by the hot cloud from the mountain. All the houses are blanketed in ash, completely white. The leaves have been burned off the trees."
Endita Sri Andiyanti, a spokeswoman at the main local hospital, said 25 people were dead and more than a dozen others were being treated for injuries.
Government vulcanologist Subandrio told the BBC's Karishma Vaswani in Jakarta that the volcanic activity appeared to have subsided based on the recordings of the levels of hot ash in the air.
However, he said he expected more eruptions soon, although there was no way of telling when or how big they would be.
Another vulcanologist, Ed Venski of the Smithsonian Museum of Natural History, told the BBC's World Today that pressure building up under a "lava dome" inside the volcano threatened further devastation.
"This the largest hazard at Merapi. It builds up as a sticky lava, where it builds up into a dome.
"At some point either there's an explosion from below that causes it to collapse, or a simple addition of lava causes it to collapse. And this sends large, hot blocks of solidified lava down the slopes," he said.
This "pyroclastic flow" is highly dangerous both in its heat and the poison of the gases.
Experts hope the volcano, some 500km (310 miles) south-east of Jakarta on Indonesia's most heavily populated island, Java, will release steam slowly rather than erupt in a big blast.
Authorities are continuing to move away thousands of local villagers living near the volcano, but it is proving to be difficult.
Many villagers ignored warnings because they were reluctant to leave homes and farms unattended.
The thousands that were evacuated are being held in makeshift emergency shelters with straw sleeping mats and bags of clothes and food.
One of the dead was a two-month-old baby. Many victims at a local hospital had severe burns.
'Three explosions'
On Monday, officials monitoring the volcano had raised the alert for Mt Merapi to the highest possible level. It erupted just before dusk on Tuesday.
A dead cow is found in Pakem village on the slopes of the volcano
Since then, more than 600 volcanic earthquakes have been recorded around the mountain.
"We heard three explosions around 1800 (1100 GMT) spewing volcanic material as high as 1.5km (one mile) and sending heat clouds down the slopes," government vulcanologist Surono told AFP.
He said this eruption was more powerful than the volcano's last blast, in 2006, which killed two people.
In 1930 another powerful eruption wiped out 13 villages, killing more than 1,000 people.
