WASHINGTON — The top United States trade negotiator said Monday that it was unclear whether Canada, Mexico and the United States could reach a deal to overhaul the North American Free Trade Agreement within the ambitious timetable set by the Trump administration.

In remarks ahead of a third round of talks beginning on Saturday in Ottawa, Robert Lighthizer, the United States trade representative, said negotiators were “moving at warp speed, but we don’t know whether we’re going to get to a conclusion, that’s the problem.”

“We’re running very quickly — somewhere,” Mr. Lighthizer said in a rare question-and-answer session at the Center for Strategic and International Studies, a Washington-based think tank.

The Trump administration has carved out a narrow path to victory on Nafta, pledging to hammer out substantial changes in a matter of months to a sweeping pact governing most of the North American economy.