We are concluding our live coverage of the huge tsunami that has devastated coastal towns on Indonesia's Sunda Strait.

Here's what you need to know:

• More than 220 people have been killed and 843 injured after the tsunami struck without warning at about 21:30 local time on Saturday
• There are fears that the death toll will rise further as emergency teams reach the hardest-hit areas
• It is thought that the Anak Krakatau volcano erupted and triggered undersea landslides which then caused the tsunami
• The government has told people to move to higher ground amid fears that a second tsunami could strike
• You can read dramatic stories from some of the survivors here.
• And read more on the science behind the tsunami here.

It is thought that undersea landslides may have caused the tsunami, but officials are still working to establish the full details.

One theory posed by experts is that part of the Anak Krakatau volcano collapsed, with material then entering the sea and pushing water ahead of it.

And these satellite images, taken by Europe’s Sentinel-1 spacecraft, may back up that explanation.

They were posted on Twitter and appear to show that the side of the volcano collapsed and spouted a cloud of ash into the sky.

Video shows the aftermath of the tsunami that ravaged parts of Indonesia's coastline.

Buildings have been flattened and people are being warned to stay away from coastal towns.

From members of a popular Indonesian rock band to a Norwegian photographer, a number of survivors have shared their dramatic stories of the disaster.

Azki Kurniawan, 16, said he was at a hotel in the popular resort area of Carita Beach on Java when people suddenly burst into the lobby yelling: "Sea water rising!"

He then ran to the parking lot to try to reach his motorbike but it was already flooded by the time he got there.

"Suddenly a 1m (3.3ft) wave hit me," he said.

You can read more firsthand accounts of the tsunami here.

Image source, Reuters

Michel Rooijackers from Save the Children Indonesia has told the BBC that the charity will work alongside the Indonesian government to respond to this disaster.

"In events like these children are much more affected than adults," he said. "So we will set up programmes around psychosocial support and make sure they are back in the classroom quickly."

"The government is well capacitated to handle situations like these and so is Save the Children," he added.

Image source, Reuters

Emergency officials are investigating whether the tsunami was caused by undersea landslides after the Anak Krakatau volcano erupted.

When volcanoes erupt, hot magma pushes underground and can displace and break through colder rock, volcanologist Jess Phoenix told the BBC.

This can trigger a landslide which then pushes the water, possibly causing a tsunami.

Jonathan Amos
Science correspondent, BBC News

If there had been a major earthquake tremor associated with the eruption on the Anak Krakatau volcano, this might have been enough to prompt many locals to take evasive action.

But although there was seismicity reported by sensitive instruments, it wasn't large enough to change people's behaviour. And you really have to rely on yourself in an evacuation in cases like this because the distance from the source of the tsunami is so short.

"Tsunami warning buoys are positioned to warn of tsunamis originated by earthquakes at underwater tectonic plate boundaries," explains Prof Dave Rothery from the UK's Open University.

"Even if there had been such a buoy right next to Anak Krakatau, this is so close to the affected shorelines that warning times would have been minimal given the high speeds at which tsunami waves travel," he says.

What an event like this one (and the one at Palu City which also caught the population unawares) teaches us is that there needs to be far more investigation into the hazards that exist away from the expected dangers in the region.

Read more from Jonathan Amos here.

Image source, AFP/ Getty Images

BBC Indonesia correspondent Rebecca Henschke describes people in the affected areas as being on edge.

They are watching and listening to the sea, she says, looking for signs of another tsunami

She says the government has told people to move to higher ground but adds that the access roads are not good.

Jonathan Amos
Science correspondent, BBC News

Nobody had any clue. There was certainly no warning. It’s part of the picture that now points to a large underwater landslide being the cause of Saturday’s devastating tsunami in the Sunda Strait.

Of course everyone in the region will have been aware of Anak Krakatau, the volcano that emerged in the sea channel just less than 100 years ago.

But its rumblings and eruptions have been described by local experts as relatively low-scale and semi-continuous. In other words, it’s been part of the background.

And yet it's well known that volcanoes have the capacity to generate large waves. The mechanism as ever is the displacement of a large volume of water.

Except, unlike in a classic earthquake-driven tsunami in which the seafloor will thrust up or down, it seems an eruption event set in motion some kind of slide. It isn't clear at this stage whether part of the flank of the volcano has collapsed with material entering the sea and pushing water ahead of it, or if movement on the flank has triggered a rapid slump in sediment under the water surface.

The latter at this stage appears to be the emerging consensus, but the effect is the same - the water column is disturbed and waves propagate outwards.

• At least 222 people have been killed and 843 injured after a tsunami hit coastal towns on Indonesia's Sunda Strait
• The tsunami waves struck at night without any warning, hitting several popular tourist destinations and destroying hundreds of buildings
• Officials say the tsunami could have been caused by undersea landslides after Anak Krakatau volcano erupted
• People have been warned to stay away from coastal areas
• No foreign nationals have been reported dead, officials say
• Read the latest developments
• Watch the destruction caused by the tsunami
• How reliable are tsunami warning systems?

Image source, AFP

Indonesia is prone to tsunamis because it lies on the Ring of Fire - the line of frequent earthquakes and volcanic eruptions that circles virtually the entire Pacific rim.

In September, more than 2,000 people died when a powerful earthquake struck just off the central Indonesian island of Sulawesi, setting off a tsunami that engulfed the coastal city of Palu.

On 26 December 2004, a series of huge waves triggered by a powerful earthquake in the Indian Ocean killed about 228,000 people in 13 countries, mostly in Indonesia.

However, tsunamis caused by volcanic activity, as the latest one is suspected to be, are less frequent.

Image source, AFP

Speaking at a news conference on the island of Java, National Disaster Management Agency spokesman Sutopo Purwo Nugroho warned that the risk of more tsunamis wasn't over:

"Recommendations from Meteorology, Climatology and Geophysical Agency (BMKG) are that people should not carry out activities on the beach and stay away from the coast for a while, because the potential for a fresh tsunami is still possible because the volcanic eruption of Anak Krakatau continues to occur, potentially triggering tsunami."

Speaking from the disaster zone, Sydney Morning Herald correspondent James Massola described people "picking through the ruins of their homes trying to find clothing and food"

However, he told BBC World News that help was getting through to survivors more quickly compared to previous natural disasters, partly due to the area’s proximity to the Indonesian capital, Jakarta.

Concerning early warnings of the tsunami, he said rescue agency officials have suggested the system was damaged by the volcanic eruption so information wasn’t getting through.

"The trip wires weren't tripped and people were surprised."

Rudi Herdiansyah, who owns a small shop by the beach in Cinangka, Baten province, Java, said the area was quiet on Saturday night.

There was no rain and everything seemed normal, he told the BBC's Indonesian service.

"Then suddenly I heard a very loud noise from the sea." The wall of water smashed into his shop, dragging everything away. He recalled being knocked down three times by the powerful wave.

"Thank God. Allah saved me, I was able to get out from the debris."

He said he didn’t hear any warnings. But he had once taken part in a tsunami drill. "It made me aware… I tried to get hold of anything to help me survive. I hid away, and clung onto a bench to be safe."

Witnesses across the areas affected by the tsunami say it hit without warning, leaving people desperate.

Azki Kurniawan, 16, said he was undergoing training with some 30 other students at Patra Comfort Hotel in the popular resort area of Carita Beach on Java when people suddenly burst into the lobby yelling: "Sea water rising!"

He told AP news agency that he wasn't sure what was happening because he didn't feel an earthquake. He ran to the parking lot to try to reach his motorbike but it was already flooded by the time he got there.

"Suddenly a 1m (3.3ft) wave hit me," he said.

"I fell down, the water separated me from my bike. I was thrown into the fence of a building about 30m from the beach and held onto the fence as strong as I could, trying to resist the water, which feels like it would drag me back into the sea. I cried in fear. 'This is a tsunami?' I was afraid I would die."

Image source, Reuters

Disaster risk reduction expert Carina Fearnley said further landslides in the areas hit by the tsunami were possible.

"It's an unstable structure. A lot of material has been shifted so definitely there is scope for further landslides," she told BBC World News.

"Generally once a landslide has happened it will settle but it might change the landscape of the volcano on the side and make it more vulnerable."

She also said scientists were working to detect where landslides can happen.

However, she added it was "very costly" to monitor all coastlines and an early warning system would be "very challenging" to establish.

Image source, Reuters

Rescue workers and ambulances are finding it difficult to reach some hard-hit areas because some roads have been partially destroyed or blocked by debris, officials say.

The western coast of Banten province in Java was the worst-hit area, according to Indonesia's national disaster agency spokesman Sutopo Purwo Nugroho.

The tsunami hit popular tourist areas around Indonesia's Sunda Strait in a busy time of the year.

ABC Australia's Indonesia correspondent David Lipson has visited a beachfront hotel where three tourists were killed.