The world's largest active volcano, Hawaii's Mauna Loa, is erupting for the first time in almost 40 years.
An ashfall advisory is in effect for the surrounding area and emergency crews have been placed on alert.
"Lava flows are contained within the summit area and are not threatening downslope communities," officials said early on Monday.
But the notification from the US Geological Service (USGS) warned the situation could change rapidly.
The volcano's alert level has also been upgraded from an "advisory" to a "warning" - the highest classification.
The latest eruption began on Sunday night at Moku'āweoweo, the volcano's summit caldera. Calderas are hollows that form beneath the summit at the end of an eruption.
It followed a series of warnings that an eruption was possible after a spate of recent earthquakes in the region, including more than a dozen reported tremors on Sunday.
"Based on past events, the early stages of a Mauna Loa eruption can be very dynamic and the location and advance of lava flows can change rapidly," the USGS said.
If the eruption migrates beyond the walls of the summit caldera, lava flows could "move rapidly downslope", it added.
Mauna Loa last erupted in March and April of 1984, sending lava flows within 5 miles (8km) of the city of Hilo.