Rim fire

A huge smoke cloud rises into the air in Tuolumne County on Tuesday as the Rim fire continued to rage. (Barbara Davidson / Los Angeles Times / August 27, 2013)

Firefighters battling the huge Rim fire were confronted with intense fire activity on two flanks and spot fires burning far outside the perimeter of the blaze, officials said Tuesday evening.

The fire, which had charred more than 287 square miles, was burning actively on the east and north perimeters as flames continued to rage into Yosemite National Park, the U.S. Forest Service said. The fire was 20% contained.

Earlier Tuesday, Tioga Road on the west side of Yosemite was closed from the Big Oak Flat entrance to Crane Flat so that emergency vehicles could drive through, park officials said. The closures are expected to last at least through Labor Day weekend.

Visitors could still enter the the park through California 41 and 140.

"Most of Yosemite National Park is not affected by the fire and is relatively smoke-free," officials said in a statement.

Firefighting operations were compounded by spot fires, or new blazes sparked by embers that ignited brush beyond the containment lines. Crews were also facing "crowning," or fires raging in the tops of large trees, according to the Forest Service.

Flames on Tuesday evening were threatening more than 5,500 structures, 4,500 of them residences, the California Department of Forestry and Fire Protection said.

More than 4,000 firefighters on nearly 500 fire engines were struggling to stop the massive blaze on the ground, Cal Fire said. They were being aided by 17 helicopters and a number of fixed-wing aircraft, including huge DC-10 bombers filled with fire retardant that made repeated assaults on the fire fronts.

Fire officials said the air attack was helpful in knocking down spot fires that broke out beyond the perimeter and aiding ground crews in protecting structures.

"Heavy reliance on aviation resources has been critical," Forest Service officials said in a statement.

The Rim fire continued to spew out huge clouds of smoke that prompted hazardous-air alerts more than 100 miles away in Carson City, Nev., officials said

The blaze was the seventh-largest in California history and had destroyed 111 structures, including 31 homes. The cause was under investigation.

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Twitter: @LAJourno

robert.lopez@latimes.com