On Saturday, giant waves crashed into coastal towns on the islands of Sumatra and Java, killing at least 281 people and injuring 1,016.
It is thought that volcanic activity set off undersea landslides which in turn generated the killer waves. Anak Krakatau erupted again on Sunday, spewing ash and smoke.
“The number of victims and damage will continue to rise,” said agency spokesman Sutopo Purwo Nugroho.
The tsunami struck at 21:30 local time (14:30 GMT) during a local holiday, with few of the warning signals that might have come had it been generated by an earthquake.
Hundreds of buildings were destroyed by the wave, which slammed into the coast of southern Sumatra and the western tip of Java about 9:30 pm (1430 GMT) on Saturday after a volcano known as the “child” of Krakatoa erupted.
The spokesman for the National Disaster Management Agency, Sutopo Purwo Nugroho, told a news conference that another tsunami is a possibility because of the continued volcanic eruptions of Anak Krakatau.
“Recommendations from [the] Meteorology, Climatology and Geophysical Agency are that people should not carry out activities on the beach and stay away from the coast for a while,” he said.
The vast archipelago nation is one of the most disaster-prone nations on Earth due to its position straddling the so-called Pacific Ring of Fire, where tectonic plates collide.
Most recently in the city of Palu on Sulawesi island a quake and tsunami in September killed thousands of people.