Firefighters battling a forest fire near Yosemite National Park since last week have manage to encircle about a third of the blaze, but a main road into the park remained cut off, officials said on Monday.

Apparently ignited by an explosion in a motor home last Thursday, the blaze has torched 4,755 acres and forced the closure of state Route 140 west of the park, said Tim Ludington, a spokesman for the fire management team.

Numerous campgrounds along the Merced River in the Sierra and Stanislaus National Forests have been shut down, as has the privately owned Cedar Lodge resort on Highway 140 in the town of El Portal, Ludington said.

The Bridalveil Creek Campground inside the park was closed to the public for use as a staging area for firefighters.

In addition, the nearby village of Rancheria was evacuated on Saturday, and residents of El Portal, most of whom work in the park, have been asked to stand by for possible evacuation.

Amazing work by the fire crews, especially that first afternoon, has resulted in no injuries, Ludington said.

He added that no structures have burned, nor have any of the 800-plus firefighters battling the blaze been hurt.

Using helicopters and airplanes as well as engines to douse the blaze, the firefighters have saved 70 residences, two commercial buildings and 35 outbuildings, he said.

Yosemite as a whole remains open, with its spectacular vistas unclouded by smoke. Really, in Yosemite Valley you would not know a fire is burning, Ludington said.

Visitors can still enter the park through State Route 120 from the east or west, and via State Route 41 from the south. Highway 140 remains open east of El Portal.

Still officials said they could not predict how far the fire would reach before they can fully contain it.

The blaze began about noon on Thursday when a recreational vehicle caught fire. Because the conflagration started simultaneously on both sides of the Merced River, officials suspect that an explosion, perhaps in the vehicle's propane tank, threw flames far and wide, Ludington said.

The so-called Motor Fire is one of several California blazes in the past week that have heated up what had been a relatively calm fire season, said Daniel Berlant, a spokesman for the California Department of Forestry and Fire Protection.

We have not seen as many large and dangerous fires as usual, he said.

The year has been relatively wet and cool, dampening the risk of fire until recently. Cool weather prevents vegetation from drying and tends to keep moisture in the air. But abundant rains also spurred heavier growth in underbrush, building up fuel. As winds picked up, the hazard was increasing, he said.