Rescue teams have been striving to reach isolated tsunami-hit areas in Indonesia, where more victims are believed to remain under the rubble.

A doll lays outside a damaged house following the tsunami in Sumur, Indonesia, Tuesday, Dec. 25, 2018. The Christmas holiday was somber with prayers for tsunami victims in the Indonesian region hit by waves that struck without warning Saturday night.[Photo:AP]

Meanwhile, the number of those displaced by the volcano-triggered tsunami rose sharply to over 20,000 after a warning of possible further giant waves, officials of the disaster agency said on Wednesday.

The catastrophe has forced a total of 21,921 people living along the coastal areas to flee home, compared with a total of 16,082 people on Tuesday, spokesman of national disaster management agency Sutopo Purwo Nugroho said.

Indonesian meteorology and geophysics agency, and volcanology agency on Wednesday morning warned that residents should avoid any activities in the radius of 500 meters to 1,000 meters from coast line amid the possibility of a second tsunami, Sutopo said.

"The hike number of the displaced person resulted from the additions of number of evacuees in several new evacuation centers," he told a press conference.

The tsunami hit at 9:27 p.m. Jakarta time (1427 GMT) on Saturday, after the eruption of the Anak Krakatau volcano sparked underwater landslides, leading to the rise of the sea level.

It struck the district of Pandeglang and Serang of Banten province and Lampung Selatan district of Lampung province, killing over 400 people and injuring at least 1,400 others so far.

The tsunami devastated a 312.75-km-long coastal area, including the province of Banten and Lampung, with most of the victims being retrieved from tourists resorts in Pandeglang district of Banten province, said Sutopo.

Sutopo said not all the rattled areas have been reached by rescuers so far, taking Sumur sub-district of Pandeglang district as an example.

"The rescue team is attempting hard to enter six villages in Sumur. Helicopters have been deployed in the mission with focus of the mission is evacuation, search and rescue for the victims," he said.

A satellite image showed the cause of the tsunami that hit the coastal areas of Sunda Strait, as most of the flank areas on the southwest of the Anak Krakatau volcano collapsed during the night, Sutopo said.

On the eruption of the volcano, the country's meteorology and geophysics agency said the flank collapse covered an area of 64 hectares and caused underwater landslides.

The Anak Krakatau is an island that emerged in 1927 and has kept growing since then. The Anak Krakatau volcano is one of the 129 active volcanoes in Indonesia, a vast archipelagic nation home to 17,500 islands and sits on a vulnerable quake-hit zone so called the Pacific Ring of Fire.

The disaster also devastated a total of 924 houses, 73 hotels and villas, 44 ships, and infrastructure facilities such as sea ports, the official said.

The height of the waters coming into onshore hovered four to five meters high, head of emergency department of disaster management agency in the district Endang Permana told Xinhua via telephone.

Separately in Aceh province of northern tip of Sumatra Island of western Indonesia on Wednesday, people marked the 14th anniversary of the 2004 Indian Ocean tsunami, which killed some 226,000 people, including 170,000 in Aceh province.