State firefighters were laboring Wednesday to restrain a stubborn fire in the steep hillsides of Poamoho Forest Reserve in central Oahu.

By Wednesday afternoon, firefighters had encircled about 90 percent of the 30-acre fire on Kaukonahua Ridge, mauka of the Helemano Military Reservation, and were trying to keep the flames from reaching a native forest nearby, said Department of Land and Natural Resources spokeswoman Deborah Ward.

She said the flames were in a gulch and not threatening any structures.

The fire burned only one acre on March 18 and was considered 100 percent contained before the weekend. But on Monday, strong winds breathed life into embers buried in deep leaf litter and the fire spread to 3 acres. By Tuesday, it had stretched out to 30 acres, scorching a eucalyptus forest with uluhe ferns and strawberry guava, Ward said.

Federal, city, and private contractors assisted state firefighters on Monday and Tuesday. Three contracted helicopters made water drops along the perimeter on Tuesday. A light rain also helped.

State firefighters were walking the perimeter Wednesday to maintain a fire break and planned to continue to do so on Thursday, Ward said.

A cause has not been determined.