BERLIN (AP) — A court has opened the door for possible bans on older diesel cars in the German city of Stuttgart, a major auto industry center, upholding a complaint by an environmental group.

The city's administrative court on Friday ordered the state government in Baden-Wuerttemberg to rework a plan to improve the air quality in Stuttgart, saying that it wouldn't bring improvements sufficiently fast, news agency dpa reported. The state has been trying to avoid bans on diesel cars thanks to automakers' pledges to retrofit vehicles.

The Environmental Action Germany group challenged a clean air plan for Stuttgart that's due to take effect in January. Nitrogen dioxide levels have long exceeded permitted levels in the city.

Friday's ruling leaves open whether, when and how diesel models might be banned.