California wildfire / Credit: ABC NEWS

A wildfire in California has exploded in size to become the largest blaze in the state so far this year.

The McKinney Fire, which started in the northern Siskiyou county on Friday, has already burnt 21,000 hectares (52,500 acres), the state’s fire service said.

At least 2,000 residents as well as trekkers on the Pacific Crest hiking trail have left the area. An unknown number of homes have been destroyed.

It was still 0% contained on Monday morning, the fire service reported.

McKinney Fire is burning in the Klamath National Forest, near the border with Oregon. Some 650 firefighters are battling the flames in punishing heat, the Los Angeles Times reports.

A red flag warning indicating the threat of dangerous fire conditions is in place, as California suffers from persistent drought conditions.

A state of emergency was declared in Siskiyou county on Saturday, after homes were destroyed and infrastructure was threatened, state governor Gavin Newsom said.

The fire was “intensified and spread by dry fuels, extreme drought conditions, high temperatures, winds and lightning storms,” he added.

Several communities are being threatened, including Yreka and Fort Jones, the US Forest Service said.