Lava flowing from Hawaii's erupting Kilauea volcano is threatening highways and officials may order thousands more people to evacuate before escape routes are cut off.
Lava from a huge new fissure is tearing through farmland towards a coastal dirt road that is one of the last exit routes for about 2000 residents in the southeast area of Hawaii's Big Island.
More lava-belching cracks are expected to open among homes and countryside about 40km east of Kilauea's smoking summit, possibly blocking another remaining exit route, Highway 132.
Fountains of magma spouted "lava bombs" over 30m into the air as the molten rock traveled east-southeast towards the coastal road, Highway 137, the Hawaiian Volcano Observatory said.
Hawaii National Guard spokesman Jeff Hickman said if either highway is hit by lava, mass evacuations would be triggered.
Since eruptions began 10 days ago, dozens of homes have been destroyed and officials have ordered the evacuations of nearly 2000 people in the lower Puna district of the Big Island, home to around 187,000 residents.
American Red Cross said 500 people sought refuge in its shelters overnight night because of worsening volcanic activity.
The number of fissures stands at 19.
"It's optimistic to think that this is the last fissure we're going to see," Hawaiian Volcano Observatory scientist Steve Brantley said.
A similar seismic event in 1955 lasted 88 days, he said.
Unnerved by near constant small earthquakes and emissions of toxic sulfur dioxide gas, Rob Guzman and his husband Bob Kirk left their home in Kalapana Seaview Estates while they still could.
"We just need the local government to calm down the panic that some of these 2000 people are feeling, that today, we're going to be trapped with no way out," Guzman said.
While residents deal with noxious gas and lava on the ground, the US Geological Survey is concerned that pent-up steam could cause a violent explosive eruption at the volcano crater, launching a 6100m plume that could spread debris over 19km.
Scientists had expected such explosions by the middle of this month as Kilauea's lava lake fell below the water table.
The possibility exists, however, that water may not be entering the crater as feared, and gas and steam may be safely venting, scientists said.
"So far those explosions have not occurred, and I think the key here is that the vent system is an open one, therefore pressure is not being built or developed down at the top of the lava column," Mr Brantley said.