US regulators have rejected Volkswagen's recall plan for diesel cars fitted with emissions "cheat" devices.
The California Air Resources Board said the proposals did "not adequately address overall impacts on vehicle performance, emissions and safety".
The board also said the proposed fix was not fast enough.
It said it would continue its investigation as well as talks with VW to find a suitable solution.
The head of the California Air Resources Board (CARB), Mary Nichols, said: "Volkswagen made a decision to cheat on emissions tests and then tried to cover it up.
"They continued and compounded the lie and when they were caught they tried to deny it. The result is thousands of tons of nitrogen oxide that have harmed the health of Californians. They need to make it right."
The federal Environmental Protection Agency (EPA) also said the VW plan for vehicles with two litre diesel engines was not acceptable.
The rejection comes ahead of a meeting between VW chief executive Matthias Muller and EPA chief Gina McCarthy on Wednesday to discuss the emissions scandal.
Volkswagen said in response: "Today's announcement addresses the initial recall plans Volkswagen submitted to CARB in December. We are committed to working co-operatively with CARB and other regulators, and we plan to continue our discussions tomorrow when we meet with the EPA."
The issue affects almost 600,000 vehicles in the United States and up to 11 million worldwide.
The scandal has severely damaged the German car maker's reputation and sparked investigations in several countries.
In the US alone VW is facing fines that could run into tens of billions of dollars.
Sales of VW brand vehicles fell by 4.8% in 2015 to 5.82 million cars - the first decline in 11 years - in the wake of the scandal.
Herbert Diess, VW chairman, said last week he was optimistic the company would find a solution soon.
"We will bring a package together which satisfies our customers first and foremost and then also the regulators," he said.
However, the EPA, which first revealed the existence of the devices, said that VW had not yet "not produced an acceptable way forward".
The US Department of Justice is suing Volkswagen on behalf of the EPA with a lawsuit that was filed on 4 January in a federal court in Detroit, Michigan.
The DoJ said the filing was the first step in "bringing Volkswagen to justice".