UPDATE 1-Top U.S. NAFTA negotiator sees no problem with pace of talks

(Releads with comments by chief U.S. negotiator)

OTTAWA, Sept 25 (Reuters) - The top U.S. negotiator at talks to modernize the NAFTA trade pact on Monday dismissed questions about why his team had so far failed to produce specific proposals on key issues, saying "I don't see a problem."

Officials from the United States, Mexico and Canada are in Ottawa for the third of seven planned rounds of talks. The U.S. delegation has yet to unveil its precise position on several points, prompting concerns the process to update the 1994 pact could drag on beyond the scheduled end-December finish.

"We've been working very hard so I don't see a problem," John Melle told reporters when pressed on the matter. "We're moving across the board, so it's very ambitious."

Canadian Prime Minister Justin Trudeau earlier predicted some tough days ahead for negotiators and declined to say whether he thought the talks could meet the deadline.

"The negotiations are still under way and of course there will be more difficult discussions in some cases than others," he told a Toronto news conference.

Asked whether he was concerned the talks might not end on schedule, he replied: "The negotiations move forward at a certain pace and we respect that reality."

U.S. President Donald Trump, who frequently describes the treaty as a disaster, is threatening to walk away unless major changes are made.

Canada's chief negotiator on Sunday said he did not expect the U.S. side to present detailed proposals in Ottawa on major issues such as dispute settlement, the dairy sector and tougher rules for North American content on autos.

Canadian officials say it is still possible to meet the year-end deadline although they concede there are significant uncertainties about the timetable.

Earlier this month, U.S. Trade Representative Robert Lighthizer said the negotiators were working at warp speed.

"I think that's accurate," said Melle.

Trudeau said Ottawa's team of officials was "moving forward in good faith" and repeated a promise to defend Canada's system of tariffs and import restrictions put in place to defend its domestic dairy sector. The U.S. industry dislikes the measures.

Kenneth Smith, Mexico's chief negotiator, told reporters late on Sunday that "we feel there is a positive environment in the negotiations."

Lighthizer, Canadian Foreign Minister Chrystia Freeland and Mexican Economy Minister Ildefonso Guajardo - the three top officials driving the NAFTA modernization - will meet in Ottawa on Tuesday and Wednesday, the last two days of the third round.

(Reporting by David Ljunggren; Editing by Paul Simao and Nick Zieminski)