WASHINGTON (Reuters) - The U.S. Trade Representative's office said on Monday it is seeking a provision to deter currency manipulation as part of its renegotiation of the North American Free trade agreement, a move that would set a precedent in a U.S. trade deal.

In a letter to lawmakers setting out negotiating objectives, USTR Robert Lighthizer said that the pact should, "through an appropriate mechanism, ensure that the NAFTA countries avoid manipulating exchange rates in order to prevent effective balance of payments adjustment or to gain an unfair competitive advantage."

Neither Canada or Mexico is currently on a U.S. Treasury a "watch list" for possible currency manipulation.

(Reporting by David Lawder)