A three-alarm forest fire burning in the White Mountains for more than a day could grow larger Wednesday, and a witness claims it was started by a meteorite, Woodstock, N.H., fire Chief John MacKay said.

The fire was first reported around 6:15 a.m. Tuesday and has extended over 22 to 25 acres in the area of Kinsman Ridge in Lost River Gorge, MacKay said.

The “fairly good-sized fire’’ has been hard to contain, he said, as it has spread across “very steep, rocky slopes.’’

“It’s not a large fire, but it’s a difficult fire because of the steepness of it, and the elevation,’’ MacKay said.

As of Wednesday morning, two helicopters and more than 50 people were working to suppress the blaze, which MacKay said could expand throughout the day.

“I expect it to grow, especially with the sun coming up,’’ he said.

No injuries or structural damages have been caused by the fire, and Lost River Gorge remains open to visitors, but the Fire Department is working to make sure the surrounding area remains safe while containment continues.

“Just in case, we are doing some structural protection,’’ MacKay said.

The cause of the fire was “undetermined’’ as of Wednesday morning, but MacKay said a passerby told firefighters Tuesday night that a meteorite might have sparked the blaze.

“Last night as we were getting out of the woods, a passerby stopped and said the night before at 7:35 p.m., he was driving by and saw a meteor streak through the sky and crash,’’ MacKay said.

A meteor is classified as a meteorite after making contact with Earth. The Fire Department is unsure as to whether the report of the cosmic fire-setting was accurate.

“We can’t say yes or no,’’ he said.

“There’s hiking trails through this area, so there is the possibility of a cigarette or a campfire,’’ he said.

MacKay is hopeful the fire will be under control later Wednesday or Thursday but was not sure when it would be fully extinguished.

“We’re hoping for containment today; if not, definitely tomorrow,’’ MacKay said. “As far as putting it out, it’s going to be more of a ‘Monitor it and hope for rain.’ ’’

Ben Thompson can be reached at ben.thompson@globe.com. Follow him on Twitter @Globe_Thompson.