An aeroplane has crashed in the El Cajon area of Southern California in the United States, authorities have said.

An unspecified number of injuries have been reported, the San Diego County Sheriff's Department reported on Twitter.

Streets are being blocked and deputies are asking drivers to avoid the area, noting it will take time to process the scene.

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The Sheriff's Department said the plane was a Learjet, and authorities are uncertain how many people were on board, CNN affiliate KGTV reported.

There was a fire, authorities said, which has been put out.

The Federal Aviation Administration (FAA) and the National Transportation Safety Board (NTSB) have been notified, and will handle the plane crash investigation, the Sheriff's Department said.

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"We are just helping with the initial response and road closures" the Sheriff's Department said in an email to CNN.

El Cajon is about 25 kilometres east of San Diego.