The disclosure took trade watchers and auto industry figures in both countries by surprise. The issue of auto tariffs had not appeared in either government’s public statement after the temporary trade-war truce.

It was unclear what China had agreed to do, if it had agreed to do anything at all. A spokesman for the Chinese Foreign Ministry referred questions to the Commerce Ministry. That ministry was silent, and its weekly news conference is not until Thursday. American officials were unavailable for comment.

The confusion is perhaps inevitable. Chinese leaders hoard information as a way of bolstering their political power. Add to that Mr. Trump’s predilection for trying to summarize complex issues in Twitter posts of a few words, and the potential for confusion can grow quickly.

China does not import many cars from the United States. American imports are roughly 1 percent of the market, or about $10.5 billion worth. While Chinese drivers love to buy Chevrolets and Fords, those vehicles are generally made in Chinese factories through joint ventures between local and American carmakers.

A concession on auto tariffs could be significant, however. It could suggest that China is willing to bend on the issue of Chinese-made cars, an area of great concern for the White House. The Trump administration worries that China could someday swamp American car lots with Chinese-made cars that could seriously damage the Detroit automakers.