The brush fire on the Big Island began in an open pasture area yesterday afternoon, according to Hawaii County fire officials, who had about 10 firefighters working to contain the blaze today.

On O'ahu, the Nanakuli fire that began Tuesday afternoon and crept into Lualualei Valley yesterday was declared contained at 4 a.m., said Capt. Emmit Kane, spokesman for the Honolulu Fire Department.

About 25 firefighters hiked into Nanakuli Valley on foot today, working with picks, shovels, chain saws and water packs to put out the remainder of the fire and stop any flare-ups, Kane said. The department's helicopter assisted with water drops.

The fire is the largest so far on O'ahu this year, said Kane.

Hawai'i's brush fire season typically starts around the end of May or beginning of June, and runs through the end of September. So far there have been more than 200 fires this year, he said.

"It's a little unsettling because it's early on in the season," Kane said. "Last year this time we had about 100, so we're up to two times as many."

The Nature Conservancy said the Nanakuli fire was threatening some 60 species of endangered native Hawaiian plants and animals in its 3,582-acre Honouliuli Preserve in the Waianae Mountains, but Kane said the fire had not encroached upon the habitat as of this morning.

Kane said fire officials suspect the Nanakuli fire's origin is suspicious and are working with police to find who many have started it. He said a resident of the area reported seeing two youths running from the scene Tuesday afternoon after the blaze started.

Federal firefighters were working to contain the blaze in Lualualei Valley, which burned about 70 acres, he said.