FILE PHOTO: U.S. Trade Representative Robert Lighthizer testifies before a Senate Finance Committee hearing on "The President's 2019 Trade Policy Agenda and the United States-Mexico-Canada Agreement" on Capitol Hill in Washington, U.S., June 18, 2019. REUTERS/Leah Millis

MEXICO CITY (Reuters) - U.S. Trade Representative Robert Lighthizer stressed that the United States has a stake in Mexico’s success as representatives from the United States, Mexico and Canada gathered in Mexico City on Tuesday to sign modifications to the United States-Mexico-Canada Agreement, a new trade pact.

New rules on the origin of steel and aluminum, a key sticking point for Mexico at the end of the negotiations, will go into effect in seven years for steel and in 10 years for aluminum, Mexican Senate Leader Ricardo Monreal said at the ceremony celebrating the signing of the deal.