OTTAWA, Ontario (AP) — Contentious issues remain in talks on renegotiating the North American Free Trade Agreement as the United States, Canada and Mexico conclude a third round of talks.

U.S. Trade Representative Robert Lighthizer says some issues related to small and medium-size enterprises were resolved, but an enormous amount of work still needs to be done on other, difficult matters.

He says delegates made significant progress on competition policy, digital trade, state-owned enterprises and telecommunications. Details on the chapters they agreed on were not available.

Mexican Economy Secretary ldefonso Guajardo also said Wednesday that "substantial challenges" lie ahead.

U.S. President Donald Trump has called the 1992 trade deal the worst in history.

Canadian Foreign Minister Chrystia Freeland says a thorough update is due.

A fourth round will be held in Washington Oct. 11-15.