WASHINGTON: Talks on the USMCA trade pact are under way with Mexico but have been more challenging with Canada, U.S. Trade Representative Jamieson Greer said on Tuesday, adding that negotiations on the bridge connecting Detroit and Windsor, Ontario, are separate.

"This negotiation is specific to the bridge," he said told Fox Business Network, referring to the Gordie Howe International Bridge. U.S. President Donald Trump on Monday threatened to bar the new US$4.7 billion bridge connecting Detroit and Windsor, Ontario from opening, in his latest salvo against Canada over trade issues.