(AFP) - Indonesian rescuers battled heavy rains Monday in a desperate search for survivors after a volcano-triggered tsunami killed at least 281 people, with experts warning the devastated region could be slammed by more tidal waves.

The national disaster agency said casualty figures were set to rise as it issued the fresh death toll -- up from a previous 222 -- and said more than 50 people were still missing.

Over 1,000 were injured in the disaster on Saturday evening, the agency added.

"The number of victims and damage will continue to rise," said disaster agency spokesman Sutopo Purwo Nugroho.

Rescue teams used diggers and other heavy equipment to haul debris from the stricken area around the Sunda Strait on Monday, as thousands were evacuated from the affected region.

"The military and police are searching the ruins to see if we can find more victims," said Dody Ruswandi, a senior official at the disaster agency, adding that the rescue effort was likely to last a week.

Hundreds of buildings were destroyed by the wave, which hit the coast of southern Sumatra and the western tip of Java around 9:30 pm (1430 GMT) on Saturday after a volcano known as the "child" of the legendary Krakatoa erupted.

The destructive wave left a trail of uprooted trees and debris strewn across beaches. A tangled mess of corrugated steel roofing, timber and rubble was dragged inland at Carita beach, a popular spot for day-trippers on the west coast of Java.

Dramatic video posted on social media showed a wall of water suddenly crashing into a concert by pop group "Seventeen" -- hurling band members off the stage and then flooding into the audience.

Two band members were killed along with its manager, while three others, including the lead singer's wife, are missing.

Dozens of victims were killed on the Sumatran coast.

- 'Saw the water coming' -

Indonesian president Joko Widodo was scheduled to visit the area Monday, less than three months after a quake-tsunami killed thousands of people around the city of Palu on Sulawesi island.

The vast archipelago nation is one of the most disaster-prone nations on Earth due to its position straddling the so-called Pacific Ring of Fire, where tectonic plates collide.

Experts have said that Saturday's rare volcano-sparked tsunami could have been caused by an underwater collapse of part of the volcano.

Anak Krakatoa, which forms a small island in the Sunda Strait between Java and Sumatra, emerged around 1928 in the crater left by Krakatoa, whose massive eruption in 1883 killed at least 36,000 people and affected global weather patterns for years.

"The likelihood of further tsunamis in the Sunda Strait will remain high while Anak Krakatoa volcano is going through its current active phase because that might trigger further submarine landslides," said Richard Teeuw of the University of Portsmouth in England.

Unlike those caused by earthquakes, which trigger alert systems, these tsunamis give authorities very little time to warn residents of the impending threat.

"It happened very quickly," said survivor Ade Junaedi.

"I was chatting with a guest at our place when my wife opened the door and she suddenly screamed in panic. I thought there was a fire, but when I walked to the door I saw the water coming."

The International Federation of Red Cross and Red Crescent Societies said the "powerful waves" reached a height of 30-90 centimetres (1-3 feet).

Oxfam and other international aid agencies said they would help in the aftermath, as local rescuers evacuated the injured and set up tarpaulins to provide shelter.

"It's already raining heavily and the winds are strong so we've only got a short timeframe to evacuate people and clean up," said Indonesia's public works minister Basuki Hadimuljono.

© Agence France-Presse