MEXICO CITY – An association of Mexican truckers said this week it is suing the United States for $6 billion in a dispute over the nation's refusal to allow Mexican haulers onto its roads as required by the NAFTA trade pact. 

Reuters reported that about 4,500 trucking companies represented by Mexico's National Cargo Transportation Association (Canacar) are involved in the lawsuit. Mexico issued punitive tariffs on $2.4 billion worth of U.S. exports in March after the U.S. Congress scrapped a 2007 program that opened up American highways to some Mexican long-haul trucks. 

The United States agreed under the North American Free Trade Agreement with Mexico and Canada to let Mexican trucks use its highways beginning in 1995. But Mexican fleets have remained confined to a narrow border area. U.S. labor unions and consumer groups have continued to argue that Mexican truck safety standards are lax and pose a threat to the American public.