Negotiators are moving swiftly to revamp the North American Free Trade Agreement but the outcome remains uncertain, US Trade Representative Robert Lighthizer said Monday.   "We're moving at warp speed but we don't know whether we're going to get a conclusion," he said.   The remarks following a speech on trade policy came days ahead of the third round of talks aimed at reworking NAFTA, which are due to kick off Saturday in Canada.   US President Donald Trump last month again called the future of NAFTA into doubt, saying he would "probably" terminate the agreement "at some point."   Despite a campaign promise to abandon the trade pact, Trump opted instead to call for talks to renegotiate it, but has signalled several times he is willing to terminate it if he does not get a good deal.   Lighthizer said Monday officials are eager to conclude the talks due to time pressures from the political calendar, notably in Mexico, which will hold general elections in July, and because uncertainty was inconveniencing businesses.   "There are a lot of people that the whole process is having real-life effects on, real farmers and ranchers and businesspeople who are trying to do business," said Lighthizer.   "So there are reasons to move quickly on a renegotiation."   Trump officials have put heavy emphasis on reducing bilateral trade deficits in all of its discussions, including with NAFTA partners Canada and Mexico, something analysts say may be difficult to remedy in trade talks.   -'Unprecedented' threat from China-   Turning to China, Lighthizer said the World Trade Organization was not equipped to respond to the "unprecedented" threat from Beijing's policies.   "The sheer scale of their coordinated efforts to develop their economy, to subsidize, to create national champions, to force technology transfer and to distort markets in China and throughout the world is a threat to the world trading system that is unprecedented," he said.   Lighthizer said he did not want to "prejudge" the outcome a pending US investigation into Chinese intellectual property policies, but there is "an awful lot to indicate there's a problem."   The Trump administration this month blocked Chinese efforts to acquire a US semiconductor firm in Oregon due to national security issues, partly because of concerns about forced transfer of sensitive technology.   Washington has also cited national security in launching probes of Chinese steel and aluminum production.