A second key route into Yosemite National Park in the US state of California has been closed as fires continue to burn.

The east entrance closure came as the so-called Rim Fire burned deeper into the park and crept closer to the tourist hub of Yosemite Valley.

The blaze stands as the sixth largest on record in state history after charring an area larger than Chicago.

It has reached a reservoir that serves as the primary water supply for San Francisco, around 320km to the west.

Crews have been attacking the eastern flank of the fire as it spread towards Yosemite Valley.

Around 4,500 homes in a string of small communities stand in the path of the flames on the western edge of the blaze.

Since beginning on 17 August, the Rim Fire has blackened about 192,500 acres of dry scrub and timberlands, mostly in the Stanislaus National Forest west of the park.

As of yesterday, 43,000 acres of the total area burned was inside Yosemite, nearly double the figure from Monday.

By late last night, containment lines had been established around 30% of the fire's perimeter, up from 23% early in the day, according to fire officials.

Treacherous, often inaccessible terrain is said to be hindering firefighters' efforts to carve additional containment lines around the blaze.

The flames last week forced the closure of a stretch of Highway 120 that leads into the west side of the 750,000-acre park and is the main entrance from the San Francisco Bay area.

Yesterday, the closure was extended along the entire length of 120, also called Tioga Road, through the park to the east gate, shutting down the second of four routes into Yosemite.

The expanded closure was imposed to allow fire crews to build containment lines along the road before the blaze approaches, Yosemite spokesman Tom Medema said.

"That will limit the access for visitors to and from the east side of the park, quite possibly over Labor Day weekend, which will have a significant economic impact on the area and (be) an inconvenience for visitors," he said.

About 4 million people visit Yosemite each year, most of them during the peak months of June through August.