NAFTA negotiators are working on a side deal for the U.S., Canada and Mexico to make a non-binding pledge to avoid devaluing their currencies for competitive purposes, according to three people familiar with the matter.

The U.S. wants to send a signal to the rest of the world that it opposes currency manipulation and prefers countries to move toward market-based exchange rates, said the people, who spoke on the condition of anonymity because the talks aren't public. The U.S. Trade Representative's office didn't immediately respond to a request for comment.

The currency pledge won't be brought into the main NAFTA agreement, according to the people. As a result, the side deal won't be as legally enforceable as the principal accord, which must be ratified by legislators in all three countries.