MEXICO CITY (Reuters) - Talks to revamp the North American Free Trade Agreement (NAFTA) should not be subject to outside pressure, Mexico's economy ministry said in a statement Thursday, and will proceed independent of U.S. President Donald Trump's tariff policy.

Trump announced on Thursday that tariffs on steel and aluminum imports will start in 15 days with initial exemptions for Canada and Mexico and the possibility of alternatives for other countries.

Trump linked the exemptions for Canada and Mexico to progress on renegotiating NAFTA to favor the United States.

"The process to negotiate a modernized NAFTA continues its course, independent of the (metal tariffs) or any other internal policy measure that the government of the United States takes," the Mexican statement said.

The statement doubled-down on similar comments earlier on Thursday that Economy Minister Ildefonso Guajardo made prior to Trump's announcement.

(Reporting by Ana Isabel Martinez)