OTTAWA — Six weeks into the renegotiation of the North American Free Trade Agreement, the thorniest issues surrounding the pact remain unresolved.

The United States, Canada and Mexico wrapped up the third round of trade talks in Ottawa on Wednesday, and negotiators from all sides said they are working nights and weekends to bring the deal to a close.

But most also freely admitted that the most difficult part of negotiations is yet to come. That includes putting to paper, for the first time, some of the most sweeping changes the Trump administration wants to the trade agreement.

Trade negotiations are typically opaque. The text being worked on is classified in the United States, and no team wants to weaken their position by giving away too much information to their opposition. But public statements at the close of the third round of talks this week offer new insight into progress on some of the main goals for the Trump administration’s renegotiation of Nafta.