Lava flowing from giant rips in the earth on the flank of Hawaii's erupting Kilauea volcano is now threatening highways, raising the possibility officials may order thousands more people to evacuate before escape routes are cut off.

Lava from a huge new fissure tore through farmland towards a coastal dirt road that is one of the last exit routes for some 2,000 residents in the southeast area of Hawaii's Big Island.

More lava-spewing cracks are expected to open among homes and countryside more than 40km east of Kilauea's smoking summit, possibly blocking one of the last exit routes - Highway 132.

Fountains of magma spouted "lava bombs" more than 30m into the air as the molten rock traveled east-southeast towards the coastal road, Highway 137, the Hawaiian Volcano Observatory said.

Mass evacuations would be triggered if either highway is hit by lava, a Hawaii National Guard spokesman said.

"There's a lot of worst-case scenarios and roads getting blocked is one of them," the spokesman said.

Dozens of homes have been destroyed since eruptions began and officials have ordered the evacuations of nearly 2,000 residents in the lower Puna district of the Big Island, home to around 187,000 people.

The American Red Cross said 500 people sought refuge in its shelters on Sunday night because of worsening volcanic activity.

Two more fissures opened in the past 24 hours, bringing the total to 19.

"It's optimistic to think that this is the last fissure we're going to see," a Hawaiian Volcano Observatory official said.

A similar seismic event in 1955 lasted 88 days, they said.

The Hawaii Fire Department issued a "condition red" alert yesterday because fissures in the southeast area of the Lanipuna Gardens area were issuing high levels of sulfur dioxide.

"Condition RED means immediate danger to health so take action to limit further exposure. Severe conditions may exist such as choking and inability to breathe," the department said in the alert.

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 3,000 metre-high 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.