MONTREAL — Discussions to revamp the North American Free Trade Agreement moved from stalemate to actual negotiation during the sixth round of talks that concluded on Monday, but a deal was still far from guaranteed as Mexico, Canada and the United States continue to squabble over how to reshape the 24-year-old pact.

Government officials and trade analysts described the mood around the talks as “cautiously optimistic” as Canada, in particular, joined Mexico in offering counterproposals to America’s requests for drastic changes, an outcome that seemed likely to dissuade the United States from imminent withdrawal.

Yet more than six months into the talks, a conclusion still appeared elusive. And tensions between the countries grew as the United States criticized Canada’s suggested changes to the pact on areas including automobile manufacturing and investment.

Robert Lighthizer, the United States trade representative, suggested that Canada had been responsible for the stalled talks. He said discussions were now progressing as the nation recognized the need to protect its trading relationships, though he added that talks were not moving fast enough.