NAFTA talks are switching gears and slowing down as key obstacles emerge, with Canada and Mexico rejecting what they see as hard-line U.S. proposals and negotiators exchanging their strongest public barbs yet.

U.S. Trade Representative Robert Lighthizer, Mexican Economy Minister Ildefonso Guajardo and Canadian Foreign Minister Chrystia Freeland wrapped up the fourth round of North American Free Trade Agreement talks in Washington Tuesday and said negotiations will run through the end of March 2018, abandoning a December target. They also extended the time between negotiating rounds, giving themselves more space to consider proposals.

The ministers cast the longer timelines as a positive way to dig into tougher disputes, pledging to continue to work out a deal while acknowledging that strong differences remain. They next meet in Mexico Nov. 17-21.