WASHINGTON—Chinese and U.S. negotiators are starting to flesh out a deal that could defuse trade tensions by boosting U.S. exports and loosening regulations that hobble U.S. firms operating in China, say people tracking the talks.

On Saturday, President Trump tweeted that he and Chinese President Xi Jinping had recently talked by phone and made “big progress” in talks, due to conclude on March 1. “Deal is moving along very well,” Mr. Trump tweeted. “If made, it will be very comprehensive, covering all subjects, areas and points...