Volcanic activity and a high tide are suspected to be responsible for the devastation on Java and Sumatra islands.

Both sides of Indonesia’s Sunda Strait – Java on the east and Sumatra on the west – were hit by a tsunami late on Saturday, causing hundreds of deaths and widespread devastation.

The tsunami, which is believed to have been caused by undersea landslides from volcanic activity, caught the region by surprise.

Here’s what we know so far:

How many were killed or injured? What is the scale of the devastation?

More than 280 people have been killed, over 1,000 injured and dozens are still missing, officials said on Monday morning.

The death toll is expected to rise.

The tsunami also damaged about 350 boats and vessels, 556 houses, 73 vehicles and nine hotels, according to Indonesia’s disaster agency (BNPB).

The majority of the deaths happened in worst-affected Pandeglang district on Java’s western tip. Serang, further north, and South Lampung, on Sumatra island, were also hit hard.

The seaside areas were packed with domestic tourists during the long holiday weekend, but foreigners were also visiting ahead of Christmas as well.

More than 220 people were killed in the tsunami [Fauzy Chaniago/AP]

What might have triggered the tsunami?

Indonesia’s climatology agency (BMKG) said undersea landslides triggered by an eruption of the Mount Anak Krakatau volcano, located about 50km offshore in the Sunda Strait between the islands of Java and Sumatra, may have triggered the tsunami.

The agency said the eruption took place at 9:03pm (14:03 GMT), and the tsunami hit 24 minutes later.

“At the same time, the tide was high due to the full moon, so it was a combination of two natural phenomenons, the tsunami and the high tide,” a spokesman for BNPB, Sutopo Purwo Nugroho, said in a statement.

Simon Boxall, oceanographer, told the Associated Press, “We know that there was a major eruption of the volcano on Friday but the tsunami that occurred was a day later. “So it was likely to be as a result of either a pyroclastic flow – these are these sort of lava flows that come off the volcano after an eruption – or more likely it was a submarine landslide, which was sort of triggered by the eruption itself. “Now, the tsunami that was created was, I say, only three metres. The problem is it occurred during a high tide, a high spring tide, which means that the water levels were already very, very high. “There was very little warning. It would have only taken about 10 minutes for the tsunami to hit the coast to the west, took about an hour to hit the east. But even so, there’s still very little time to get a warning out and there are no buoys in this particular area for early warnings.”

Is this the first tragedy of its kind in the area?