The Indonesian volcano Anak Krakatau, part of the Krakatoa system, has erupted.

Reports say it sent a plume of ash several kilometres into the air, with a boom that could be heard from far away.

"Please stop making that booming noise and go to bed, Anak Krakatau. It's late and we've already got plenty of other things to worry about," one person wrote on Twitter.

"Krakatau is erupting nonstop for 2 hours," wrote another. "I live in bogor and I can hear the noises as clear as everybody else hear. This feels like a nightmare."

Volcano tracking site Volcano Discovery called it a "large magmatic eruption", at least the biggest since 2018, which caused a 150m-high tsunami wave that hit an uninhabited island. That boom was so big, Anak Krakatau lost two-thirds of its height.

"Both the webcam on Anak Krakatau Island and from the coast (in 40 km distance) show strong lava fountaining from the volcano," Volcano Discovery said, saying the plume was 14km high.

A local expert told CNN Indonesia the lack of traffic at the moment, thanks to COVID-19 lockdown measures, made it easier to hear Krakatoa's rumbles.

Magma Indonesia said the "main crater smoke is gray and black with moderate to thick intensity about 200-500 meters from the summit".

When Krakatoa erupted in 1883, it created a tsunami that was still a whopping 42m high when it hit land, killing 36,000 people.