NEWHALEM, Wash. (AP) — A wildfire has forced a major cross-state highway to close in Washington’s rugged North Cascades National Park.

State Route 20, also known as the North Cascades Highway, was closed Friday night between Newhalem and Rainy Day Pass by the state Department of Transportation. It remained closed Monday with no estimated reopening date.

The Sourdough fire ignited on July 29 because of a lightning strike near Diablo in the steep terrain of the Ross Lake Recreation Area. The fire on Monday continued slowly moving toward the Diablo Dam, a Seattle City Light power plant and an area where park workers live, the Bellingham Herald reported.

“The fire hasn’t gone all the way down to those structures. We still want to treat them as if they are at risk,” Mark Enty, a spokesperson for the National Interagency Fire Center team that’s now managing firefighting operations, told the newspaper.

The North Cascades Highway is the northernmost pass connecting eastern and western Washington. It’s closed to ensure firefighter safety as they use the road to move equipment, Enty said.

Motorists can use U.S. Highway 2, Interstate 90 or U.S. Highway 12 as alternative routes across the state, the Department of Transportation said.

The blaze had burned about 2.25 square miles (5.8 square kilometers) as of Monday, according to fire officials.

Firefighters are protecting the town of Diablo, the power-generating facilities and the campus of the North Cascades Environmental Learning Center, he said.

About 340 firefighters and support personnel are working on the fire with helicopters playing a big role in slowing the fire’s spread, according to the National Interagency Fire Center.

The North Cascades National Park Visitor Center and Newhalem area campgrounds remain open, officials said, but people should be prepared for possible wildfire smoke.