Indonesia on Thursday raised the danger level for an island volcano that triggered a tsunami over the weekend, killing at least 430 people in Sumatra and Java, and widened its no-go zone.

The volcanology agency raised the Anak Krakatau volcano's alert status to the second highest and more than doubled the exclusion zone to a 5-km radius.

The eruption on Saturday evening caused part of the island in the Sunda Strait to collapse into the sea, apparently generating tsunami waves of more than 2 metres. Most tsunamis are caused by earthquakes.

The government has warned Sunda Strait communities to stay a km away from the coastline because of the risk of another tsunami triggered by Krakatau's ongoing eruptions.

'There's still a chance of a landslide, even under the sea level or on the sea level,' said Rudy Sunendar, head of the energy ministry's geology department.

He said that bad weather was preventing a closer look at the volcano. 'Based on the satellite imagery interpretation, there is collapse of some area of Mount Anak Krakatau,' he told The Associated Press at the volcano's monitoring post.

Saturday's disaster struck without warning, taking people by surprise in a country that regularly suffers landslides, earthquakes and volcanic eruptions. No big earthquake shook the ground beforehand, and the waves surged inland at night on a holiday weekend while people were enjoying concerts and other beach and resort activities.

A tsunami survivor walks through the ruins of a villa with her husband in Carita, Indonesia, on Wednesday. AP

Indonesia's tsunami warning system relies on land seismometers and buoys connected to tidal gauges, and is not equipped to detect underwater landslides. The system, in any case, has not operated for years because the buoys have suffered from a combination of vandalism and lack of funds for maintenance.

Heavy rains and high seas have hampered the search effort. Some bodies were found at sea and at least 159 people are missing.

On Thursday, residents of badly affected Banten province in Java were combing through the debris of destroyed or damaged homes for anything salvageable.

'I've lost everything I have, my house and all belongings inside it,' said farmer Muhamad Sarta.

'I just hope for some help from the government,' he said. 'Hopefully there will be some repairs. I have nowhere to go. I have no money. Whatever I had was lost in the water.'

Radar data from satellites, converted into images, shows the Anak Krakatau volcano is dramatically smaller following Saturday's eruption.

Satellite photos aren't available because of cloud cover but radar images from a Japan Aerospace Exploration Agency satellite taken before and after the eruption show the volcano's southwestern flank has disappeared.

JAXA's post-eruption image shows concentric waves radiating from the island, which experts say is caused by its ongoing eruptions.

Dave Petley, head of research and innovation at Sheffield University who analyzed similar images from a European Space Agency satellite, said they support the theory that a landslide, most of it undersea, caused the tsunami.

'The challenge now is to interpret what might be happening on the volcano, and what might happen next,' he wrote in a blog.

Anak Krakatau, which means child of Krakatau, is the offspring of the infamous Krakatau volcano that affected the global climate with a massive eruption in 1883.

Anak Krakatau first rose above sea level in 1929, according to Indonesia's volcanology agency, and has been increasing its land mass since then.