The Californian forest fire that is threatening some of the oldest and largest trees in the world has grown to 57,000 acres (22,800 hectares).

However, fire crews said they had been able to save several groves of giant sequoia trees from the flames and spokesman Jim Paxon described Thursday as a "good day for firefighters".

Some sequoias are up to 10 metres wide at their base

About 1,500 firefighters are battling the fire in and near a forest known as Giant Sequoia National Monument, using water-carrying aircraft, helicopters and bulldozers to try to block its expansion.

Their task has been hampered by high winds and dry conditions.

So far, none of the area's sequoia stands have been destroyed, but flames have come within two miles of the Trail of 100 Giants, a grove that includes some of the Earth's largest and oldest trees.

Sequoias can live for more than 3,000 years and their thick trunks are resistant to fire, but they are vulnerable when flames reach the crowns of smaller trees and leap from there into their limbs.

A 45-year-old woman, Peri Van Brunt, is in custody on suspicion of starting the fire after losing control of a campfire while cooking hot dogs.

Campfires were not banned in the area at the time, but the fire danger had been considered extreme and permits were required. Forest rangers said Ms Van Brunt had not obtained such a permit.

Series of fires

Fires have ripped through several parts of the United States and Canada in recent months.

Firefighters in Colorado battling a blaze near Rocky Mountain National Park believe the 1,760-hectare (4,400 acre) blaze will be contained by Saturday, and crews from the area have been sent elsewhere.

But other major fires continued to burn in Oregon and Washington State.

And authorities are investigating why 20 firefighters in Oregon fighting a forest blaze had to climb into their emergency shelters to escape being overrun by flames, with several suffering smoke inhalation and minor burns.