A federal judge has given Volkswagen and regulators a month to provide a specific plan about how they will bring nearly 600,000 diesel cars into compliance with clean air laws following an emissions cheating scandal.

Senior US District Court Judge Charles Breyer said he wants to know the timing of the fix and any planned payments to owners, among other details, by April 21.

"I would hope by the 21st that as many astounding issues as possible will be wrapped up," he said.

Judge Breyer said former FBI director Robert Mueller told him Volkswagen, government regulators and attorneys for car owners had made substantial progress toward a resolution that would get the polluting cars off the road. He did not provide any details. Judge Breyer appointed Mueller to oversee settlement talks.

The parties were not able to immediately announce the solution because engineering technicalities and other important issues still needed to be resolved, Judge Breyer said Mr Mueller told him.

Volkswagen acknowledged in September that it intentionally defeated emissions tests and put dirty vehicles on the road. The cheating allowed cars to pass laboratory emissions tests while spewing harmful nitrogen oxide at up to 40 times the level allowed when operating on real roads.

The Department of Justice has sued Volkswagen on behalf of the US Environmental Protection Agency (EPA). The company also is facing lawsuits from angry car owners, who are demanding Volkswagen buy the vehicles back. Those cases are both before Judge Breyer.

Judge Breyer warned that he would seriously consider holding a trial this summer if a concrete proposal to resolve the ongoing pollution did not emerge by April 21.

The judge told Volkswagen attorney Robert Giuffra last month that he wanted to know by Thursday whether the company had come up with a fix that was technologically feasible and acceptable to the EPA. Six months was long enough to determine whether there was an engineering fix for the vehicles, Judge Breyer said at the time.

But he appeared pleased with the progress report he received on Thursday.

Volkswagen said in a statement after the hearing that it is committed to coming to a fair and prompt resolution of the diesel emissions lawsuits and was fully cooperating with Breyer.

Justice Department spokesman Mark Abueg declined to comment.

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