Hawaii County Mayor Harry Kim wrote on Twitter Friday that "fast moving lava from Fissure 20 has crossed Pohoiki Rd near Malama Ki Place," near the Puna forest reserve.
The lava is "isolating appx 40 homes in area," he wrote, adding that four people were "safely evacuated" by Hawaii County and National Guard helicopters.
"If u r still in area stay in safe place & await official instructions," Kim urged.
The US Geological Survey (USGS), which monitors volcanos and earthquakes worldwide, said on its website that "a fast-moving pahoehoe lava flow that emerged from fissure 20... continues to flow southeast," with the quickest of three "lobes" progressing at 230 yards (210 meters) per hour.
Pahoehoe, a Hawaiian word, refers to smooth, undulating lava flows.
The USGS added that "smoke from burning vegetation as lava flows advance is also contributing to poor air quality."
"A short-lived explosion" from Kilauea's Halemaumau crater "created an ash cloud that reached up to 10,000 ft" (3,000 meters) above sea level, the USGS said.
Kilauea is the world's most active volcano and one of five on Hawaii's Big Island.
It started erupting on May 3, prompting about 2,000 people to flee from their mountainside homes.
Scientists believe the volcanic activity may be a precursor to a major eruption similar to the one that shook the island in the mid-1920s.
But they say they expect no loss of life given that the most exposed residential areas have been evacuated and the region where the volcano is located -- on the southeastern part of the island -- is lightly populated.
So far, about 40 homes and other buildings have been destroyed by the red-hot lava that has been flowing from the volcano since early May.
Authorities have warned residents to stay away from the neighborhoods evacuated, warning that toxic sulfur dioxide seeping out of nearly two dozen fissures could prove deadly.
A violent explosion within Kilauea's Halemaumau crater on Thursday -- the biggest since the volcano became more active on May 3 -- produced a volcanic plume that reached 30,000 feet into the sky.
