WASHINGTON — The trade conflict between the United States and China has become so caustic that the two countries cannot even agree if they are talking.

President Trump jolted markets and left world leaders scratching their heads on Monday when he said at the Group of 7 summit in France that senior Chinese officials had phoned his advisers over the weekend to engage in trade negotiations. The alleged overtures would have come hours after Mr. Trump raised tariffs on Chinese imports and labeled President Xi Jinping an “enemy” of America in a tweet.

China said the calls never happened.

The back and forth capped a whiplash-inducing 72 hours in the trade war as Mr. Trump alternately suggested he was doubling down in his fight against China while offering hope that trade negotiations were continuing.

It also left Mr. Trump’s top economic aides straining to back up the president without contradicting him. Pressed about the calls on Monday before a meeting with Prime Minister Narendra Modi of India, Mr. Trump said that Liu He, China’s vice premier, had reached out to his team. The president called on Treasury Secretary Steven Mnuchin, who was sitting next to him, to verify the call.