Anak Krakatau in the Sunda Strait began a continuous eruption at 11.07pm local time on Friday that lasted about 25 hours, throwing up a fountain of lava with a boom reported heard 700 kilometres away.

Eight airports, two of them serving Jakarta, halted operations over the weekend after ash was detected in their airspace. At four airports managed by InJourney Airports alone, some 2,300 flights were disrupted and 268,539 passengers affected.

Several airports remained closed in the early hours of Monday because of volcanic ash. The volcano continued with moderate, intermittent eruptions, dark ash emissions and small localised earthquakes.

The head of Indonesia's Geology Agency said the probability of further eruptions in the coming days remained high, with volcanic bombs, heavy ashfall and possible lava flows named as the main hazards should activity be sustained.

Volcanic ash can severely damage aircraft engines, so suspensions follow forecasts of ash dispersal along flight paths. Airspace therefore often takes longer to return to normal than the eruption itself takes to weaken.

Anak Krakatau grew during the twentieth century inside the caldera of Krakatoa, known for its great eruption of 1883. In 2018 a collapse of its flank triggered a tsunami with heavy loss of life.

No casualties have been reported in this episode. Authorities are maintaining an exclusion zone around the crater, and travellers using regional Southeast Asian routes are advised to check flight information.