Indonesia has raised the alert status of the Anak Krakatoa volcano to the second-highest level on Thursday after a series of eruptions, Channel NewsAsia reported. All flights around the volcano have been rerouted and authorities have imposed a 5-km exclusion zone, according to BBC.

The alert comes days after a portion of the volcanic Anak Krakatoa island fell into the ocean, causing a tsunami which hit the coasts of Indonesia’s Java and Sumatra islands on Saturday night. Over 400 people died and at least 150 people remain missing after the tsunami.

The volcano has been rumbling on and off since July but has been particularly active since last week.

National disaster agency spokesperson Sutopo Purwo Nugroho said the volcanic activity of Anak Krakatoa located in the Sunda Strait “continues to increase”. “To that end, the Volcanology and Geological Disaster Mitigation Centre has upgraded the status of Anak Krakatoa volcano from Level 2 (alert) to Level 3 (standby),” he said. Indonesia has a four-level alert system.

“We have developed a monitoring system focused specifically on the volcanic tremors at Anak Krakatoa so that we can issue early warnings,” said Indonesia’s meteorology agency head Dwikorita Karnawati, according to Reuters.

Air traffic control agency AirNav Indonesia said it was closing flight routes around the volcano as the ash from it meant the situation was on “red alert”. At least 25 flights were affected, including international ones to and from Australia, Singapore and Asia.

Rescue efforts continue

Meanwhile, rescue teams have struggled to reach remote areas that were struck by the tsunami on the western coast of Java as authorities have warned of an “extreme weather” rain warning, reported Reuters. Indonesia’s meteorology agency said the rough weather could make the Anak Krakatoa volcano’s crater more fragile.

Disaster agency spokesperson Sutopo Purwo Nugroho said a state of emergency has been declared till January 4. Search and rescue teams are now focusing on the town of Sumur near the southwest tip of Java.