The incident happened on Mount Salak, a volcano in west Java popular with visitors from Jakarta at weekends.

"Four died on the spot and two died on the way to hospital," said Sudaryono from the Indonesian Red Cross hospital in Bogor, West Java.

Three other students were affected by the fumes, he said.

The nine students had left the group they were camping with about a kilometre down the slope late on Saturday.

Two were discharged on Sunday and the third was no longer in a critical condition but still being treated in hospital, Sudaryono said.

One of the survivors, 14-year-old Naimah, told the Pikiran Rakyat daily that she saw one of the girls in the group collapse while they were in the crater and lose consciousness.

"Sarifah suddenly collapsed and fainted. I wanted to help," Naimah said, adding that she smelled the strong odour of sulfur before fainting herself.

Hiking up Indonesia's many volcanoes is a popular activity across the archipelago nation.

Indonesia sits on the Pacific "Ring of Fire" where continental plates collide, causing frequent seismic and volcanic activity.

Seven climbers were found dead on the slopes of Indonesia's second highest volcano, Mount Rinjani, in March. They likely died of exposure.