At least three other hikers were also severely injured in the Sunday accident on the slope of Lawu Mountain, said Yatmo, a spokesman for the local Search and Rescue Agency.
The bodies of the victims are being evacuated to nearby hospitals, added Yatmo, who uses a single name.
The victims were among more than a dozen hikers who were caught in a fire that raced through forests on the 3,265-meter (10,712-feet) mountain located on the border between East and Central Java provinces.
Another hiking group of 13 senior high school students was evacuated safely, the Detik.Com website reported. It three women were among the dead.