The last sign of life came on Monday evening, when one of the journalists contacted a relative. They had arrived at the waters around the Anak Krakatau volcano, which has been spewing lava and ash since Friday.
An intensive search and rescue operation is now underway. As of Wednesday, there was still no trace of the group.
The volcanic eruption had major consequences for air traffic in the area. More than 340,000 travellers are reported to have been affected.
In 2018, an eruption of Anak Krakatau caused a tsunami that killed at least 430 people on the islands of Java and Sumatra.
