People watch Mount Krakatau, left, and Anak Krakatau, right, from Carita beach in Indonesia. Indonesian authorities asked people to avoid the coast in areas where a tsunami killed more than 420 people, in a fresh warning issued on the anniversary of the catastrophic 2004 Asian earthquake and tsunami. Picture: Achmad Ibrahim/AP

Jakarta - Indonesian authorities on Thursday raised the alert level of an erupting volcano, days after it triggered a tsunami that killed hundreds of people.

The alert level for Mount Anak Krakatau was raised to the second highest level, "standby," and the exclusion zone around it was also expanded to a five-kilometre radius, Indonesia's disaster mitigation agency (BNPB) said.

The volcano island, located in the Sunda Strait between Java and Sumatra, spewed clouds of ash on Wednesday that were blown over to the cities of Serang and Cilegon in Banten province, spokesman Sutopo Purwo Nugroho said.

"However, the volcanic ash is harmless," Nugroho said. "It will fertilize the soil, but residents are advised to wear masks and protective glasses when outdoors."

An eruption on Saturday resulted in the collapse of the volcano's southwestern flank, which sent huge amounts of earth down into the sea and resulted in an undersea landslide that triggered a tsunami.

The tsunami hit coastal areas of the provinces of Lampung and Banten which surround the strait, killing at least 430 people and displacing thousands.