JAKARTA, Indonesia — Residents along Indonesia’s Sunda Strait coast were warned on Monday to avoid beaches for at least two days for fear that volcanic activity could generate another powerful tsunami.

Experts were still uncertain what caused a 10-foot-high tsunami on Saturday, which killed at least 373 people and injured more than 1,459 on the nearby islands of Java and Sumatra. Nearly 12,000 people have been displaced and at least 128 are missing.

But evidence was mounting that the deadly wave resulted when volcanic activity on the island of Anak Krakatau set off a landslide on the island’s steep southwest slope.

The volcanic island, whose name means Child of Krakatau, has grown over the last century from the crater of Krakatau, or Krakatoa, and has been erupting almost daily since June.

Scientists said satellite images showed that a significant chunk of the island’s southwest flank, visible before the tsunami, had vanished after the giant wave struck the coast. That could indicate, scientists said, that a large amount of soil may have slid into the ocean, displacing enough water to create the tsunami.

“We need to determine whether a landslide actually occurred there,” said Eko Yulianto, a tsunami expert at the Indonesian Institute of Sciences.

It was the second time in three months that a deadly tsunami has hit Indonesia without advance warning for the stricken communities. An earthquake and tsunami that struck Sulawesi island on Sept. 28 killed more than 2,100.

Indonesia’s president, Joko Widodo, toured damaged areas along the Sunda Strait on Monday, some of which had been quickly cleared of debris before his visit.

In Java’s Banten Province, the hardest hit area, he talked with survivors at a local clinic and at a camp set up to house those who lost their homes.

“I want to see firsthand the handling of the post-tsunami that swept the coast around Sunda Strait last night,” he told reporters.

Officials said that no alert had been sounded in advance of Saturday’s tsunami because Indonesia’s warning system detects only tsunamis that are caused by earthquakes.

Mr. Eko said tsunami detection buoys might have provided a warning on Saturday and saved many lives, but that part of the country’s alert system was abandoned in 2012 after some of the buoys were hit by ships and others went missing.

Costas Synolakis, director of the Tsunami Research Center at the University of Southern California, said that a tsunami generated by volcanic activity on Anak Krakatau should not have come as a surprise. The island was formed after the massive eruption of Krakatoa in 1883, one of the largest in recorded history.

“It was well known that Krakatoa was again in a paroxysm and tsunamis from submarine landslides triggered by eruptions were not unexpected,” he said. “What was unexpected was that local residents were apparently not forewarned to be extra vigilant for unusual water motions as the volcano was erupting.”

He said tide gauges placed in the Sunda Strait could have given some people enough warning to flee the coastline.

Even posting lifeguards on the beach to watch for unusual water motion could have given some people time to flee.

“Warnings from lifeguards allow at best for a few extra minutes for evacuations, but of course they are better than no warnings at all,” he said. “With hindsight, it is clear that a few extra tide gauges around the islands in Sunda Strait would have saved lives, even without offshore buoys.”

The death and injury toll was relatively high because many people were at the beach on Saturday evening enjoying a long holiday weekend. Many others lived close to the beach, especially on Java, a densely populated island with more than 140 million people.

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