BUENOS AIRES—The U.S. and China, looking to defuse tensions and boost markets, are exploring a trade deal in which Washington would hold off on further tariffs through the spring in exchange for new talks looking at big changes in Chinese economic policy, said officials on both sides of the Pacific.

The talks have been conducted, via telephone, for several weeks, and are coming to a head shortly before President Trump and Chinese President Xi Jinping meet for dinner on Saturday at the end of the Group of 20 leaders summit...