The so-called Rim Fire blazing through eastern central California expanded by 50 square miles since Friday and continues to threaten the perimeter of Yosemite National Park. It is now said to be five percent contained as of Saturday, an improvement from two percent containment on Friday.

Some backwoods hiking trails and roads leading into the national park have been closed due to the fire, but the Yosemite Valley, which is home to iconic landmarks such El Capitan and the Half Dome rock formations, remains unthreatened and is still open to the public.

The fire began a week ago in Stanislaus National Forest and rapidly grew in size to become one of the largest active wildfires in the US. The blaze grew exponentially throughout the week, rising from sharply from 16,000 acres Wednesday to 54,000 acres Thursday. By Friday morning the blaze stood at 105,620 acres, doubling in size overnight Thursday.

As of Saturday evening the blaze encompassed 125,620 acres, according to the Incident Information System (InciWeb), the online system that keeps track of wildfires in the Western US.

Gov. Jerry Brown expanded the state of emergency he declared over the fire region to include the city of San Francisco, some 200 miles away from the blaze. The move came after the wildfire encroached upon utilities which provide resources to the west coast city. The San Francisco area gets 85 percent of its water from the Hetch Hetchy reservoir, which is only 4 miles away from the fire zone, The Associated Press reported. Two of the three hydroelectric power stations in the reservoir have had to be shut down, the AP said.

There are more than 2,600 fire crews participating in fire-fighting efforts, InciWeb reported.

"A major effort focused is underway to hold the fire east of the North Fork of the Tuolumne River. Fire crews have strengthened fire lines that are holding above the community of Pine Mountain Lake. Additional efforts are focused on the eastern edge of the fire in Yosemite National Park to minimize impacts to our national treasure," InciWeb said is its Aug. 24 update on the Rim Fire.

The extremely difficult terrain upon which the fire burns exacerbates the blaze's potential for growth, which InciWeb also reports as "extreme."

"The Rim Fire continues to exhibit very large fire growth due to extremely dry fuels and inaccessible terrain. Rapid fire growth and extreme fire behavior are drastically hampering suppression efforts. Aerial resources are being effective with MAFFS and DC-10 VLAT air tankers prepping locations in advance of the fires spread towards the Highway 108 corridor," InciWeb reported.

Approximately 4,500 structures are threatened by the fire, which continues its eastward spread and is well into the northwest corner of Yosemite National Park.