General Motors' quarterly sales in China reportedly fell for the first time in over a year, hit by faltering economic growth and a wider slowdown in the world's biggest auto market amid a trade war with the US.

Reuters said GM sales fell 14.9% to 835,934 vehicles in the third quarter which the automaker said was due to a "softening" vehicle market and issues changing to new engines for the Buick brand.

"The major reasons are a softening market, slowing lower-tier cities, Buick's engine change-over and a strong Q3 last year," a Shanghai-based GM spokeswoman told Reuters, adding the fall was not linked to trade tensions.

GM has been shifting its Buick cars to a new type of more efficient three cylinder engine to meet emissions targets which, Chinese dealers told Reuters, had hit sales because consumers were not yet convinced by the smaller engines.

"Many consumers still have concerns because they read negative comments about three-cylinder technology online, which aren't really fair," a sales manager at a Buick dealer in Zhejiang province told the news agency.