The governor of California, Jerry Brown, has declared a state of emergency in response to a major gas leak that has forced thousands of people from their homes on the outskirts of Los Angeles.
Since October, huge quantities of methane gas have been escaping from an underground gas storage facility in Aliso Canyon in the city's north-west.
Local people have complained of headaches, nausea and other health problems, and thousands relocated over Christmas.
The Governor was calling for alternatives to stop the natural gas leak if existing efforts failed.
Southern California Gas Co, which operated the well that was the site of the methane leak, was 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 would need to identify how it will stop the leak if the relief well failed to seal it - or if the existing leak worsened.
The leak was discovered on 23 October at a well used for natural gas storage in Aliso Canyon just outside Los Angeles' Porter Ranch neighborhood in the San Fernando valley, 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 have said they expect to stop the leak in late February to late March.
State officials said the leak at its peak accounted for a fourth of California's 20 million metric tons a year in greenhouse gas emissions from methane.
The leak was believed to have been caused by a broken injection-well pipe several hundred feet beneath the surface of the 1,457-hectare field.
People in the area have complained of such ailments as headaches, nausea and respiratory irritation from mercaptans, the odorants added to natural gas, according to Los Angeles County health officials.
They have said past studies found no long-term health effects from mercaptans.
The governor's office said it had taken steps to coordinate the local, state and federal response to the leak, which at last count was releasing more than 30,300 kilograms of methane per hour, and stop it as soon as possible.
The emergency declaration by Jerry Brown, a Democrat, followed calls by local officials in Los Angeles for increased state regulation of gas storage facilities.
The emergency declaration made clear Southern California Gas was responsible for covering costs from the leak and allowed the governor's administration, if necessary, to quickly waive laws in the future to deal with the crisis.
Residents of Porter Ranch have said they want the gas field shut down.
For his part, the governor called for regulators to assess the long-term viability of natural gas storage facilities in California.
President and chief executive of the company Dennis Arriola said SoCalGas stood ready to cooperate with the governor's office, all state and local officials and regulatory agencies.
The leaking storage field in Aliso Canyon is the second largest such facility in the western United States by capacity.