A tsunami has killed at least 222 people and injured hundreds on the islands of Java and Sumatra following an underwater landslide believed caused by the erupting Krakatoa volcano, officials and media say.
Hundreds of homes and other buildings were "heavily damaged" in the tsunami which struck along the rim of the Sunda Strait late on Saturday.
TV images showed the seconds when the tsunami hit the beach and residential areas in Pandeglang on Java island, dragging with it victims, debris, and large chunks of wood and metal.
It has been the latest in a series of tragedies that have struck Indonesia, a vast archipelago, this year. Successive earthquakes flattened parts of the tourist island of Lombok, and a double quake-and-tsunami killed thousands on Sulawesi island. Nearly 200 people died when a Lion Air passenger plane crashed into the Java Sea in October.
Authorities warned residents and tourists in coastal areas around the Sunda Strait to stay away from beaches and a high-tide warning remained in place through until Christmas Day.
"Please do not be around the beaches around the Sunda Strait. Those who have evacuated, please do not return yet," said Rahmat Triyono, an official at the Meteorology, Climatology and Geophysics Agency (BMKG).
President Joko Widodo, who is running for re-election in April, said on Twitter that he had "ordered all relevant government agencies to immediately take emergency response steps, find victims and care for the injured".
Vice-President Jusuf Kalla told a news conference the death toll would "likely increase". Neighbouring Malaysia and Australia both said they were ready to provide assistance if needed.
At least 299 New Zealanders in Indonesia at present
The New Zealand embassy in the Indonesian capital Jakarta is in contact with local authorities but says there's no information to suggest any New Zealanders have been caught up in the tsunami.
A spokesperson for the Ministry of Foreign Affairs and Trade says there are currently 299 New Zealanders registered on SafeTravel as being in Indonesia.
The Ministry is urging New Zealanders in the area affected by the tsunami to follow the advice of the local authorities, and to let friends and family know their whereabouts.
Rescue workers and ambulances were finding it difficult to reach affected areas because some roads were blocked by debris from damaged houses, overturned cars and fallen trees.
Endan Permana, head of the disaster mitigation agency in Pandeglang, told Metro TV police were providing assistance to victims in Tanjung Lesung in Banten province, a popular tourist getaway not far from the capital, Jakarta, as emergency workers had not arrived.
The western coast of Banten province in Java was the worst-hit area so far, said Sutopo Purwo Nugroho, spokesman for the national disaster mitigation agency.
At least seven people were reported dead in Lampung in southern Sumatra.
Stage washed away
Around 250 employees of the state utility company PLN had gathered in Tanjung Lesung for an end-of-year event, company spokesman I Made Suprateka told Reuters. At least seven people were killed and many suffered broken bones, he said.
Dramatic TV footage showed the seconds when waves hit a concert at the event and washed away the stage where local rock band Seventeen was performing.
"The water washed away the stage which was located very close to the sea," the band said in a statement. "The water rose and dragged away everyone at the location. We have lost loved ones, including our bassist and manager... and others are missing."
The disaster mitigation agency said it was still compiling information on the disaster and there was a "possibility that data on the victims and damage will increase".
The tsunami was caused by "an undersea landslide resulting from volcanic activity on Krakatoa" and was exacerbated by abnormally high tide because of the full moon, disaster agency spokesman Mr Nugroho said.
Ben van der Pluijm, an earthquake geologist and a professor at the University of Michigan, said the tsunami may have been caused by a "partial collapse" of Krakatoa.
"Instability of the slope of an active volcano can create a rock slide that moves a large volume of water, creating local tsunami waves that can be very powerful. This is like suddenly dropping a bag of sand in a tub filled with water," he said.
Krakatoa, an active volcano which is located roughly halfway between Java and Sumatra and has been spewing ash and lava for months, erupted again just after 9pm on Saturday and the tsunami struck at around 9.30pm, according to BMKG.
Krakatoa is the island that emerged from an area once occupied by Krakatau, which was completely destroyed in an 1883 disaster. It first appeared in 1927 and has been growing ever since.
Coastal residents reported not seeing or feeling any warning signs, like receding water or an earthquake, before waves of up to two metres washed ashore, according to media.
Mr Nugroho told Metro TV that tsunamis triggered by volcanic eruptions were "rare" and that the Sunda Strait tsunami had not resulted from an earthquake.
"There was no earthquake, and the Anak Krakatau eruption also wasn't that big," Mr Nugroho told Metro TV, noting there were no "significant" seismic tremors to indicate a tsunami was coming.
The eruption created a column of volcanic ash up to 500 metres high.
Footage posted by the head of the disaster management agency showed the aftermath of the tsunami, with flooded streets and an overturned car.
Data sementara dampak tsunami di Pantai di Kab Pandeglang, Serang dan Lampung Selatan hingga 23/12/2018 pukul 04.30 WIB: tercatat 20 orang meninggal dunia, 165 orang luka-luka, 2 orang hilang dan puluhan bangunan rusak. Data korban kemungkinan masih akan terus bertambah. pic.twitter.com/6f7buuoD5Y— Sutopo Purwo Nugroho (@Sutopo_PN) December 22, 2018
He had earlier posted footage of water rushing in and local residents trying to flee in panic.
Indonesia is prone to earthquakes because it lies on the Ring of Fire - the line of frequent quakes and volcanic eruptions that circles virtually the entire Pacific rim.
Civil Defence says no tsunami warnings have been issued for New Zealand.
The 1883 eruption of Krakatoa was one of the deadliest volcanic eruptions in the world, killing 40,000 people.
On Boxing Day in 2004, an Indian Ocean tsunami triggered by an earthquake killed 226,000 people in 13 countries, including more than 120,000 in Indonesia.
- Reuters / BBC