Indonesia says the volcano continues to spill lava, hours after the initial eruption. The explosion came two years after a similar eruption caused a tsunami along the Sumatra and Java coasts that killed 430 people.
Indonesia's Anak Krakatau volcano spewed a column of ash 500 meters (1,640 feet) into the sky, the country's volcanology agency said on Saturday.
Closed-circuit TV showed the lava flares on Friday night, while the agency said several hours later that the eruption was continuing.
No casualties were reported and a level 2 alert status remained in place, the second-highest on a scale of four.
Friday's eruption came two years after a huge flare-up caused a tsunami along the coasts of Sumatra and Java, killing 430 people. Thousands were injured.
The 2018 eruption caused the volcano to lose much of its visible height.
Anak Krakatau, which means Child of Kratakau, is located between the islands of Java and Sumatra and was created after the legendary eruption of Krakatau volcano in August 1883.
The eruption was so violent that the volcanic island that had existed until then sank into the sea. More than 35,000 people lost their lives.
The sea between Sumatra and Java, like the rest of the island nation, sits on the Pacific Ring of Fire, the most geologically active zone on earth where volcanic eruptions and earthquakes are frequent.
mm/bk (AP, dpa)