China's auto sales rose just 2.5 percent year-on-year in September, the slowest growth for any month this year, an industry group said Monday, as a weak economy took a toll on the world's largest car market.

Total sales hit 1.98 million vehicles for the month, the China Association of Automobile Manufacturers said in a statement.

In August, auto sales reached 1.72 million vehicles, a year-on-year rise of 4.0 percent.

Although the group cited rising seasonal demand from the traditionally slow summer months, it added year-on-year growth was "relatively low" in September.

Passenger car sales alone rose 6.4 percent to 1.70 million vehicles in September, the association said.

For the first nine months of the year, sales of vehicles -- both passenger and commercial -- rose 7.0 percent to 17.0 million units, it said.

Last year, auto sales in China reached 21.98 million vehicles, helped by a recovery in Japanese brands which were hit by a political row between Tokyo and Beijing over disputed islands.

China's economy grew 7.5 percent year-on-year in the second quarter this year, improving marginally from 7.4 percent in the first quarter, which was the worst since a similar 7.4 percent result in July-September 2012.

At the same time, a government crackdown on corruption and an austerity campaign have hurt the luxury auto segment, analysts say.

The government has also taken aim at foreign auto makers, which dominate the market, for what authorities claim is monopoly actions over prices of parts and complete vehicles, fining some.

Still, US auto giant General Motors (GM) posted its best September ever in China with sales rising 15.2 percent year-on-year to 319,936 units, according to the company.

GM expects to sell a record number of cars in China, its biggest market, this year, exceeding last year's tally of 3.16 million, GM China President Matthew Tsien said last month.