The U.S. and Canada missed a Friday deadline imposed by the Trump administration to revamp the North American Free Trade Agreement, but agreed to extend talks rather than rip up the quarter-century-old treaty.

After four days of marathon talks ended with significant differences remaining, President Trump said he still planned to stick with the timetable he laid out earlier this week to sign a new pact in late November to replace the three-nation accord he has branded “a disaster.”

But Mr. Trump didn’t define the precise shape of that pact, leaving open the possibility of a two-way accord with Nafta’s other partner, Mexico, unless Canada adopts his vision of a fairer agreement. Mr. Trump on Monday struck a deal with Mexico on a redone Nafta and then pressed Canadian Prime Minister Justin Trudeau to join by the week’s end.

“I intend to enter into a trade agreement with Mexico—and with Canada if it is willing, in a timely manner,” Mr. Trump told Congressional leaders in a Friday letter.

It’s unclear precisely what the new timetable is for Canada to join. The two sides said talks would resume on Wednesday, and people familiar with the process said that new deadline could extend to Sept. 30. That’s when, according to U.S. law, the Trump administration would need to make public final text for a trade pact that he could sign by Nov. 30, when Mexico’s leader leaves office.