A 6.8 magnitude earthquake has struck southern California, one of the most powerful to hit the region in nearly 20 years.

The US Geological Survey said the quake, initially reported as a magnitude 6.6, was very shallow – only 8.7 km (5.4 miles) – which would have amplified its effect.

The tremor, which was felt as far as Los Angeles, was quickly followed by several smaller aftershocks in the area.

The earthquake lead to some fires in Ridgecrest, a town near the epicentre of the quake.

British tourists Naomi Woodford and Alex Nelson, who were in Los Angeles, said they were advised not to use the city’s metro.

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“I was having breakfast in downtown LA then noticed the windows of a building shaking and everyone else started pointing,” Ms Woodford said. ”We heard others saying their bedroom was shaking and they were scared.”

Seismologist Dr Lucy Jones said the tremor was the strongest earthquake in southern California since 1999.