A bush fire broke out at a park in the heart of Los Angeles, causing officials to evacuate the city's historic Griffith Observatory.

Los Angeles Fire Department declared the four-acre blaze a "major emergency" and were tackling the flames from the ground and by air.

Park rangers were called in to ban hikers from trails in the area, which is popular with tourists and residents of the nearby upmarket Los Feliz neighborhood.

Griffith Park is a sprawling and rugged expanse of countryside criss-crossed by hiking and riding trails, home to Griffith Observatory, as well as Los Angeles Zoo.

The landmark observatory, opened in 1935, is world-famous and has appeared in many Hollywood films such as the James Dean movie "Rebel Without a Cause" and La La land.

The news evoked memories of a major fire in 2007, which ripped through 800 acres of the park before it was contained.

The cause of the latest fire was not known, and no homes had been evacuated by mid-afternoon.

Wildfires are a natural phenomenon in the western US, but their frequency and ferocity has increased in recent years as the planet warms.

Human behaviour, including the unchecked burning of fossil fuels, is altering weather patterns, exacerbating droughts in some areas and generating unseasonal storms in others.

Southern California is in the grip of a multi-year drought.

In California, average temperatures during the summer are 1.6C higher than at the end of the 19th century.