(New throughout, adds comments by U.S. Trade Representative, details, background)
OTTAWA, March 28 (Reuters) - U.S. Trade Representative Robert Lighthizer on Wednesday expressed optimism that talks to modify NAFTA could be wrapped up quickly but a top Canadian official was more downbeat, saying much work remained.
The United States is trying to speed up the slow pace of negotiations on the $1.2 trillion North American Free Trade Agreement, citing a need to wrap up before a July 1 presidential election in Mexico.
"I'd say I'm hopeful -- I think we are making progress. I think that all three parties want to move forward, we have a short window, because of elections and things beyond our control," Lighthizer told CNBC television.
"But if there's a real effort made to try to close out and to compromise and do some of the things we all know we should do ... I'm optimistic that we can get something done in principle in the next little bit."
Officials from the three countries are supposed to meet in the United States next month for the eighth round of talks, although Washington has not announced dates yet.
Only six of the roughly 30 chapters have been closed and wide differences remain on sensitive topics such as dispute resolution and how much North American content should be contained in autos produced in the three NAFTA nations.
Chief Canadian negotiator Steve Verheul, asked by reporters whether a deal was close, replied: "No, we've got quite a bit of work do yet".
Asked whether a deal could be done in April, he replied: "That would be a bit of a challenge."
Verheul was speaking to reporters on the sidelines of a meeting with union representatives from Canada and Mexico to discuss NAFTA.
Canadian Prime Minister Justin Trudeau last week said a deal was likely while stressing the challenges posed by the Mexican presidential election on July 1 and U.S. congressional elections set for November.
Trudeau spoke to Trump on Monday and "underscored Canada's commitment to conclude a modern, mutually beneficial NAFTA as soon as possible," according to a statement issued by the prime minister's office later the same day. (Additional reporting by David Lawder in Washington; Editing by Chizu Nomiyama and David Gregorio)