US authorities say flows of lava remained "contained" within the summit area of Mauna Loa, but the eruption could pose a threat to nearby residents should conditions change.
Emergency crews went on alert as Hawaii's Mauna Loa, the largest active volcano in the world, erupted for the first time in nearly 40 years, authorities in the United States have said.
Flows of lava remained "contained" within the summit caldera — the large depression at the mouth of a volcano — of Mauna Loa, but the eruption could pose a threat to nearby residents should conditions change, the United States Geological Survey (USGS) reported at 11:45 pm local time Sunday (9:45 GMT on Monday), some 15 minutes after the eruption inside the Hawaii Volcanoes National Park.
"At this time, lava flows are contained within the summit area and are not threatening downslope communities," the USGS said on its website, noting that residents of the area should review preparedness procedures.
Mauna Loa has erupted 33 times since 1843, according to the USGS.
The most recent eruption, in 1984, lasted 22 days and produced lava flows which reached to within about seven kms (four miles) of Hilo, a city which is home to about 44,000 people today.
While the latest eruption on the main island of the remote US state in the Pacific remains confined within the basin at the top of the volcano, called the caldera, "if the eruptive vents migrate outside its walls, lava flows may move rapidly downslope," according to the USGS.
Hours later on Monday morning, the USGS volcano monitoring office tweeted: "Lava does seem to have flowed outside the caldera, but for now the eruptive vents remain confined to the caldera."
The agency said the Hawaiian Volcano Observatory was in consultation with emergency management personnel and its staff would conduct an aerial reconnaissance over the 13,674-foot (4,168-metre) volcano as soon as possible.
Hawaii authorities said no evacuation orders have been given, although the summit area and several roads in the region were closed.
A USGS webcam on Mauna Loa summit's north rim showed long bright eruptive fissures within the volcanic crater, contrasted against the dark of night.
The Hawaiian islands are home to six active volcanoes.