California Governor Jerry Brown declared a state of emergency following a gas leak that sent huge clouds of methane into the skies.

The gas leak in Los Angeles has been sickening nearby residents for more than two months.

Footage handed out by the Environmental Defense Fund showed infra-red footage of the gas leak.

Southern California Gas Co, which operates the well that is the site of the underground methane leak, is trying to stop it by drilling a relief well to reach a damaged pipeline, then injecting fluids and heavy mud into it.

The governor's office said in a statement the utility will need to identify how it will stop the leak if a relief well fails to seal it - or if the existing leak worsens.

The leak was discovered on October 23 at a well used for natural gas storage in Aliso Canyon just outside Los Angeles' Porter Ranch neighbourhood, which is home to more than 30,000 people.

Thousands of residents were forced to move over the holidays, with the company underwriting their temporary housing.

Officials from Southern California Gas, a division of Sempra Energy, say they expect to stop the leak in late February to late March.