WASHINGTON—Prospects for a speedy conclusion to the U.S.-China trade fight dimmed Monday after U.S. officials accused Beijing of reneging on its promises and vowed to implement President Trump’s threat to raise tariffs quickly on Chinese imports.

“Over the course of the last week or so, we’ve seen an erosion in commitments by China, I would say retreating from commitments that have already been made, in our judgment,” said U.S. Trade Representative Robert Lighthizer, declaring tariffs on $200 billion in Chinese imports would...