A forest fire burning through a popular hiking area on the New Jersey-Pennsylvania border is about 80% contained.

State fire officials said Monday the blaze started Sunday and had burned about 80 acres of Worthington State Forest and the Delaware Water Gap National Recreational Area by Monday afternoon. They said it's still not clear what sparked the fire.

Controlled burns were set Sunday night to try to contain the fire. But officials said that the location made it difficult for crews to fight.

"We sent some crews in the area, our helicopter worked the flames until dark. It's pretty steep and rugged. It's probably the steepest terrain in the entire state of New Jersey,” said incident commander Eric Weber.

Some delays were reported during the Monday morning commute, but officials said the highway is open in both directions.

The Appalachian Trail runs through the area and is also open. Fire officials warned that smoky conditions will likely remain until at least Tuesday.