A massive gas leak near Los Angeles that forced thousands from their homes for months was brought under control on Thursday, a utility official said.

The methane leak, the biggest in California's history, has been deemed an environmental disaster by several outside experts.

More than 4,500 families living in the affluent Porter Ranch area were forced to relocate after the leak was detected in October as residents reported getting ill from the noxious fumes spewing out of a damaged pipe.

Chris Gilbride, a spokesman for Southern California Gas Company (SoCalGas), told AFP that a relief well on Thursday had reached the stricken well and that workers had pumped mud and heavy fluid to plug the flow of gas.

He said the next step would be to pump cement into the damaged well over several days in order to permanently seal it.

"There is no more gas flowing from the well now," he said, adding that state regulators would eventually carry out inspections to ensure the leak was over.

The company said that once the well is permanently sealed, residents forced out of their homes would be able to gradually return to the area.

Thursday's breakthrough was welcomed by area residents whose lives were disrupted by the leak and who have expressed concern about the environmental and health impact.

"Most of the families in the community are very excited to get back, but they will not be distracted by the leak stoppage," Paula Cracium, head of the Porter Ranch Neighborhood Council, told the Los Angeles Times. "They still want to know that their homes are safe."

The gas leak over the past three months has spewed greenhouse gases that will significantly contribute to global warming, experts have warned.

Methane, which is colorless and odorless, is a much more potent greenhouse gas than carbon dioxide.

Officials attribute the headaches, nausea and nosebleeds reported by Porter Ranch residents to the odorant added to methane so it can be detected.

But public health officials and the gas company -- a division of Sempra Energy -- insist the fumes pose no long-term health risks.

More than two dozen lawsuits have been filed against SoCalGas in relation to the leak which could cost the company billions of dollars.

A lawsuit filed by California's attorney general last week, alleging that SoCalGas violated state health and safety laws by failing to promptly contain the leak, seeks unspecified civil penalties.

It follows similar action taken in December by the Los Angeles city attorney.

Many Porter Ranch residents have also joined a class action suit against the gas company.