Mount Agung in northeastern Bali has been rumbling to life intermittently since late last year and it began belching smoke and ash again on Thursday.
The seismological, volcanic and meteorological institute Insivumeh advised the civil aviation authority to take precautions with flights amid renewed activity from the peak.
Residents of the Big Island were warned to take shelter from the ash as toxic gas levels spiked in a small southeast area where lava has burst from the ground during the two-week eruption.
An aviation red alert means a volcanic eruption is under way that could spew ash along aircraft routes, the US Geological Survey (USGS) says on its website.
The alert remains just one notch below the highest level of 5 after five more episodes of "intense but sporadic lava fountaining" from the summit crater over a 19-hour period from Tuesday morning.
A giant mushroom-shaped cloud shot up from the Philippines' most active volcano on Monday, darkening the skies and raining ash on communities where tens of thousands have fled after warnings ...
Ash spewed as high as 2.1 km above the crater during one emission at 7:59 am, within a radius of about 5 km around the mountain, the Indonesian Disaster Mitigation Agency said.
A rumbling volcano in the central Philippines shot a huge column of ash into the sky on Saturday, prompting the official aviation agency to warn aircraft to stay away, authorities said.