General Motors Co. and Ford Motor Co. both posted steep sales declines in China last month on a year-over-year basis, part of a wider slowdown attributed in part to a drop-off around the Lunar New Year holiday.

Sales for both U.S. car makers fell 9% in February, the companies said, following a string of monthly gains driven by new government subsidies introduced late last year to stimulate demand for fuel-efficient cars.

China’s auto market has bounced back from a slump in the summer of 2015 due to the incentives, which can be applied to 70% of cars sold in the country. But the recent sales declines raise a potential red flag, signaling the world’s largest new-car market could be permanently cooling amid the country’s slowing economic growth.

Through the first two months of 2016, GM and Ford sales rose 11% and 18%, respectively, the companies said. Analysts typically look at January and February sales together to account for the disruption caused by the New Year holiday. The China Association of Automobile Manufacturers will report official February new-vehicle sales in China for all car makers later this month.

Other auto makers also posted declines in February, including Hyundai Motor Co. and Mazda Motor Corp. Sales for SAIC Corp., China’s largest domestic auto maker, dropped 7%, dented by declines reported by joint-venture partners GM and Volkswagen AG , both market leaders in the country.

China car-sales reached a new high in 2015, rising 7.3% from a year earlier to 24.6 million. But the growth rate was slower than the double-digit gains recorded in 2013 and 2014.

The China auto makers’ association projects passenger-car sales in 2016 will expand 7.8% to 22.76 million. Car makers have rushed to build factories and boost production in China, hoping to tap surging demand for new cars created by a rising middle class and rapid urbanization in what is considered one of the industry’s most profitable markets outside the U.S.

January was a particularly strong month for auto makers in China, with sales up 9.3% from a year earlier, as buyers snapped up new-cars before the holiday. Travel tends to be heavy around the holiday, contributing to a decline in showroom traffic last month.

“It’s like a vacuum effect in February,” said Nigel Griffiths, chief automotive economist for researcher IHS Automotive. March results will be the real test to see whether demand created by the government stimulus is starting to fizzle, Mr. Nigel said.

IHS Automotive has issued a cautious forecast for new-car sales in China this year, with growth likely to benefit only certain auto makers. Demand is also starting to shift to local brands with a number of global auto makers posting weaker sales of late.

Ford has 4% of the market in China and recently completed a $5 billion expansion to build new factories and add models. The Dearborn, Mich., auto maker also plans to spend another $1.8 billion on research and development there. GM, Ford’s crosstown rival and among the largest sellers in China, is also trying to increase market share with new models and an expanded lineup of Cadillac luxury vehicles.

“It is a very densely crowded market,” particularly on factory capacity, that can dent sales and profits across auto makers, said Bill Russo, a managing partner with consultants Gao Feng Advisory.

Government tax incentives are helping to prop up new-car demand in China this year but the subsidies only last through December, analysts say.

Write to Christina Rogers at [email protected]

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