The group was climbing on Mount Lawu when the accident took place on Sunday, local disaster agency official Agung Lewis told AFP.
“Seven of them were killed and two others are in critical condition as they sustained around 50 percent burns,” he said, adding all the dead and injured had been brought down from the mountain.
“The area is actually closed for hiking because there had been previous cases of wildfires due to the dry weather, so we suspect these hikers could have used unofficial routes,” he added.
He did not give any further details about those killed or injured.
The blaze was thought to have been caused by a campfire started by hikers and not properly extinguished, said disaster agency spokesman Sutopo Purwo Nugroho.
While nobody else has been reported missing, rescuers are combing the area to check for others who might be stuck on the mountain.
Wildfires are a frequent occurrence during Indonesia’s months-long dry season.