US President Donald Trump has threatened once again to withhold federal aid from California after its Democratic governor criticized his environmental policies.

A fast-moving wildfire roars through a Northern California town of US yesterday, with deaths reports as it burns a hospital, hundreds of homes and sent tens of thousands of people fleeing flames in cars and on foot, authorities say.

Authorities hope weaker winds are going to help more than 10,000 firefighters battle the deadliest blazes in California history, which have killed at least 40 people and destroyed thousands of structures in one of the state's worst natural disasters in years.

The governor of California declares a state of emergency after wildfires forced thousands of people to flee their homes in the north of the state.