RESIDENTS in parts of Hawaii are being warned that lava could cut off a major highway as a flow heads towards the ocean.

The lava had mostly been splattering out of the Big Island volcano since a fissure opened earlier this month with nearly 2000 people forced to evacuate as lava claimed 40 structures.

But the flow changed dramatically on Friday afternoon as one fissure expanded, destroying four more homes and isolating residents, some of whom had to be airlifted.

By Saturday morning, two of 22 fissures had merged, creating a wide flow advancing at rates of up to 274 metres per hour.

Aerial footage showed fast-moving lava advancing to the southeast, extending to within 1.5 miles of the ocean.

If lava threatens main roads, more people will be told to prepare for voluntary evacuation.

A lava flow was less than a mile away from Highway 137 and would reach it in a matter of hours, officials warned.

More eruptions from the summit are possible. “We have no way of knowing whether this is really the beginning or toward the end of this eruption,” said Tom Shea, a University of Hawaii volcanologist. “We’re kind of all right now in this world of uncertainty.”