JAKARTA, Dec. 27 (Xinhua) -- Indonesian authorities have risen alert status of Anak Krakatau volcano and enlarged the no-go zone after the eruption of the volcano triggered a tsunami in coastal areas of Sunda Strait which killed over 430 people, disaster agency officials said on Thursday.

The level of danger of the volcano was raised up to the second highest from the third highest and the evacuation zone was widened to five km from the crater compared with the previous 2-km size, spokesman of the national disaster management agency Sutopo Purwo Nugroho said.

The eruption of the volcano on Dec. 22 triggered landslides which culminated the rise of the sea surface and spewed lava into the sea water, the spokesman said.

The authorities thus urged people to use masks.

Volcanologists said the volcano has experienced "a strombolian" eruption type since July, meaning that the volcanic activity produces frequent moderate eruptions.

Volcanologists and disaster authorities have warned the local community and tourists not to enter the radius of one-km from the coast along the Sunda Strait amid fear of a second tsunami.

A massive evacuation has been underway, which involves around 20,000 evacuees based on Wednesday's figures.

Saturday's tsunami devastated a 312.75-km coastal areas in provinces of Banten and Lampung.

Most victims were retrieved in several resorts in Pandglang district of Banten province, as at least 1,495 people were injured, according to the disaster agency.

A satellite image has unveiled that most of the flank areas at the Anak Krakatau collapsed just before the tsunami and the country's meteorology and geophysics agency said the flank collapse covered an area of 64 hectares and caused landslides underwater.

Anak Krakatau is one of the 129 active volcanoes of Indonesia, a vast archipelagic nation sitting on a vulnerable quake-hit zone called the Pacific Ring of Fire.