(Reuters) - Shares of U.S. automakers General Motors Co and Ford Motor Co skidded on Wednesday after a Chinese official warned the government could slap a penalty on an unnamed U.S. automaker for monopolistic behavior.

The warning from a senior Chinese state planning official, conveyed through the official China Daily newspaper on Wednesday, came days after U.S. President-elect Donald Trump questioned the longstanding U.S. policy of acknowledging that Taiwan is part of “one China.”

Chinese officials had been investigating the pricing practices of automakers prior to Trump’s comments, sources said.

Trump’s rhetorical challenges to long-established policy toward China have rattled U.S. corporations. Relying on stable U.S.-China relations for more than 40 years, the companies have ramped up sales in China and developed complex supply chains that feed Chinese-made parts to their U.S. operations.

Jason Miller, a spokesman for the transition team, said on Wednesday that members of the team were aware of the China Daily report but that it would be premature to comment. He said he expected Trump to discuss the matter with billionaire investor Wilbur Ross, who has been tapped to be commerce secretary, when the two meet on Wednesday.

“The president-elect has made very clear that he’s going to get out there and fight for American companies and American jobs, and that’s something he has not been shy about doing so far and it’s not something that we’re going to be shy about going forward,” Miller said.

While Chinese officials did not directly link the warning about a possible penalty of a U.S. automaker to Trump’s comments, the Chinese government has used regulatory sanctions against foreign corporations during previous episodes of diplomatic discord.

GM GM.N shares slumped 2.2 percent, while Ford F.N was down nearly 1 percent as the wider Dow Jones industrial average .DJI slipped.

In a statement on Wednesday, GM did not say directly whether it was under investigation by Chinese authorities. “GM fully respects local laws and regulations wherever we operate,” the company said. “We do not comment on media speculation.”

A spokesman for Ford’s Asia-Pacific operations said the company was “unaware of the issue.”

The China Daily quoted Zhang Handong, director of the National Development and Reform Commission’s (NDRC) price supervision bureau, as saying investigators had found that a U.S. auto company had instructed distributors to fix prices starting in 2014.

Cars stand bumper to bumper in the evening rush hour traffic jam in central Beijing, China, December 8, 2016. REUTERS/Thomas Peter/File Photo

The China Daily article did not give further details and the NDRC did not respond to requests for comment.

There are many ways that China could retaliate against the United States, including leveraging its holdings of $1.19 trillion of U.S. treasuries in September.

In an editorial, the China Daily urged Trump to recognize the importance of close economic ties between China and the United States.

“For the American economy to be great again ... the U.S. needs to cement its economic relations with China, rather than destroy them.”

China, the world’s largest vehicle market, is crucial to GM. Chinese consumers bought more than one-third of the 9.96 million vehicles GM sold globally in 2015. Profits from Chinese operations, including joint ventures, accounted for about 20 percent of GM’s global net income of $9.7 billion in 2015.

Ford’s China joint ventures represented about 16 percent of its global pretax profit of $9.4 billion in 2015.

“This action is just a hint as to how much power China wields,” said Michael Dunne, president of Dunne Automotive and a veteran of the Chinese auto industry. “A small fine of several million dollars is likely. The message is, ‘If you want to play, we can play.’”

While Zhang’s comments to the China Daily appeared just days after Trump’s remarks, people familiar with the situation said Chinese officials have been cracking down on what they have called monopolistic behavior by foreign automakers and dealers for several years.

If the NDRC levies a new penalty it would be the second this month and the seventh issued to automakers since the commission began anti-monopoly investigations in 2011, the China Daily said.

Targeted firms have included Audi AG NSUG.DE, Daimler AG's DAIGn.DE Mercedes-Benz and Toyota Motor Corp 7203.T, and one of Nissan Motor Co Ltd's 7201.T joint ventures.

“I don’t think the NDRC had only made a decision two weeks ago or a week ago. This is a long-term plan for them,” a source at a government-affiliated industry association said.

Analysts said on Wednesday that a separate Chinese action would undercut confidence in the Chinese auto market outlook.

Separately, China will extend a tax cut on small-engine vehicles to 2017, two sources told Reuters, a move that could prevent a predicted steep drop-off in sales.