China's auto sales expanded at the slowest pace this year in September as weak sales of commercial cars weigh on growth.

Approximately 1.98 million cars were sold in China last month, a year-on-year rise of 2.5 percent, while output increased 4.2 percent to 2 million units, the China Association of Automobile Manufacturers (CAAM) said in a report published Monday.

In the first nine months, output and sales stood at 17.22 million and 17 million, respectively, up 8.1 percent and 7 percent respectively, but growth was significantly down from the rates seen during the same period last year.

The CAAM attributed the slowdown to sluggish performances in commercial cars. Last month, sales of commercial cars totalled 287,600 units, down 16 percent from a year earlier.

The CAAM made an earlier forecast that market demand for automobiles would hit 23.83 million units this year, representing a 8.3-percent rise from 2013.

Bucking the broader trend, sales of new energy cars saw strong growth on the back of government support to the industry. In the first nine months, a total 38,163 new energy cars were sold in China, 2.8 times the volume seen during the same period last year, the CAAM said.

China has rolled out a set of measures to promote the use of new energy vehicles in its bid to save energy and combat pollution, including tax exemptions, subsidies for car purchases and requirement for government organs to buy more new energy cars.