BEIJING—China is beginning to flesh out details of a weekend tariff truce with the U.S., after days of vague Chinese statements and a barrage of comments from President Trump and other administration officials.

China’s Commerce Ministry in a statement Wednesday acknowledged for the first time that Beijing on Saturday agreed to a 90-day cease-fire to allow negotiations to take place. The statement, attributed to an unnamed spokesman, said that the negotiations have a “clear timeline and road map” and that China aims to quickly...