San Francisco  - A forest fire that has blanketed Yosemite National Park in smoke threatens around 4,000 buildings near the California natural attraction, local media reported Tuesday.

Four days after the blaze broke out about 20 kilometres from Yosemite, firefighters had contained about 10 per cent of the place, which has destroyed 40 buildings, including 12 homes.

Nearly 3,000 firefighters have been deployed and were supported by aircraft dropping water and fire retardant.

Yosemite remained open to visitors despite a thick, smokey haze, but one of the main entrance roads had been closed to traffic on Monday.

Most guests had left over the weekend. Park hotels were operating on back-up power sources and had distributed flashlights to the remaining guests. Authorities had switched off the power to protect firefighters from possible electrocution.

The fire is reaching Yosemite during the high point of the tourism season.

Since the end of June the west coast state has already suffered from enormous forest fires. Fires have charred more than 3,400 square kilometres. (dpa)