By Paul Vieira and Amanda Coletta

OTTAWA--U.S. Trade Representative Jamieson Greer said retaliatory measures from Canada on U.S. products are a major hurdle thwarting talks between the two countries on a renewed trade pact.

In an interview with Fox Business on Tuesday, Greer said senior USTR officials are conducting formal negotiations with Mexico, ahead of a July 1 date in which a formal review of the U.S.-Mexico-Canada trade treaty, or USMCA, begins. The U.S. could decide to either renew the treaty for another 16 years, or decline renewal and subject the trade deal to annual reviews for a decade--at which time the pact would expire unless there is an agreement among the three North American countries.

Unlike Mexico, Canada "has a different approach to the United States," Greer said. "They have some retaliatory tariffs still in effect, and that makes it a problem for us to negotiate," he added.

He didn't specify which tariffs posed a problem for Washington. Representatives of the USTR didn't respond to questions seeking more details.