TUOLUMNE CITY, Calif. (AP) — Officials say fire crews made progress overnight against a large wildfire threatening San Francisco’s water supply, several towns near Yosemite National Park and historic giant sequoias.

Stanislaus National Forest spokesman Jerry Snyder said containment of the Rim Fire was at 15 percent on Monday morning, up from 7 percent the previous night.

The fire did continue to grow, however, and is now 234 square miles in size.

Snyder said crews are being helped by the fire’s movement into less forested areas and cooler temperatures caused at least in part by the shadow cast by the large plume of smoke from the blaze.

About 4,500 structures and Hetch Hetchy Reservoir, the source of San Francisco’s famously pure drinking water, remain under threat.

THIS IS A BREAKING NEWS UPDATE. Check back soon for further information. AP’s earlier story is below.

One of the largest wildfires in California history loomed over San Francisco’s water supply, several towns near Yosemite National Park and historic giant sequoias on Monday as it crackled through tinder-dry forest.

Inaccessible terrain, strong winds and bone-dry conditions have hampered firefighters’ efforts to contain the Rim Fire, which began Aug. 17 and has consumed nearly 225 square miles. Officials estimate containment at just 7 percent.

It continues burning in the remote wilderness area of Yosemite and is edging closer to the Hetch Hetchy Reservoir, the source of San Francisco’s famously pure drinking water, park spokesman Tom Medema said. San Francisco gets 85 percent of its water from Hetch Hetchy as well as power for municipal buildings, the international airport and San Francisco General Hospital. The threat to the city’s utilities prompted Gov. Jerry Brown to declare a state of emergency for San Francisco.

Despite ash falling like snowflakes on the reservoir and a thick haze of smoke limiting visibility to 100 feet, the quality of the water piped to the city 150 miles away is still good, say officials with the San Francisco Public Utilities Commission.