* Emerging market stocks tumble, lead global stocks down
* U.S.-Canada talks, China tariffs deadline loom
* MSCI world stock index down for fourth day in a row (Updates after U.S. market open, adds commentary, changes byline, previous dateline LONDON)
By Sinéad Carew
NEW YORK, Sept 5 (Reuters) - Stock markets around the world fell on Wednesday and declines in emerging markets accelerated as a deadline in the U.S.-China trade conflict loomed and U.S.-Canada trade talks were due to resume.
Weakness in emerging market currencies helped support the dollar, even as sterling rebounded off a two-week low on hopes of a breakthrough in Brexit talks. That optimism came after a Bloomberg report said Britain and Germany were prepared to drop a key sticking point in the Brexit negotiations.
U.S Treasury prices gained slightly as emerging market stock weakness boosted demand for low risk U.S. government debt, though pressure from a heavy corporate debt supply calendar limited gains.
A public comment period on the possibility of fresh U.S. tariffs on another $200 billion of Chinese goods ends on Thursday, with expectations that the additional levies will be imposed by U.S. President Donald Trump.
The United States and Canada will also resume discussions on Wednesday on revamping the North American Free Trade Agreement (NAFTA). Ottawa is not expected to back down on key issues despite Trump's threats to retaliate.
"There's still a cloud hanging over the market about whether we are actually going to see these (Chinese) tariffs take effect by the end of this week and continued concerns about the NAFTA deal," said Scott Brown, chief economist at Raymond James in St. Petersburg, Florida.
The Dow Jones Industrial Average fell 9.67 points, or 0.04 percent, to 25,942.81, the S&P 500 lost 13.95 points, or 0.48 percent, to 2,882.77 and the Nasdaq Composite dropped 109.57 points, or 1.35 percent, to 7,981.68.
The pan-European FTSEurofirst 300 index lost 0.98 percent and MSCI's gauge of stocks across the globe shed 0.71 percent.
In Asia, expectations of U.S. tariffs sent the Shanghai SE Composite index down 1.7 percent.
Emerging market stocks lost 1.73 percent. MSCI's broadest index of Asia-Pacific shares outside Japan closed 1.8 percent lower, while Japan's Nikkei lost 0.51 percent.
Emerging market currencies showed a second day of declines, with a JPMorgan emerging market currency index falling 0.3 percent.
Sterling was last trading at $1.2897, up 0.33 percent on the day.
Measured against a basket of currencies, the dollar index fell 0.19 percent, with the euro up 0.26 percent to $1.1611.
"Until last month, people were focusing on U.S. company earnings but now they are looking closely at what's happening in emerging markets, at the trade war and the fact that the United States is likely to implement another wave of tariffs against China," said Christoph Barraud, an economist at Paris-based brokerage Market Securities.
"If you look at global growth, more and more signs are that it will slow in coming months."
Oil prices fell as a U.S. Gulf tropical storm weakened and moved away from oil-producing areas, and concern about weakening global demand added downward pressure.
U.S. crude fell 1.2 percent to $69.03 per barrel and Brent was last at $77.43, down 0.95 percent on the day.
Additional reporting by Shreyashi Sanyal in Bengaluru, Sujata Rao in London, Shinichi Saoshiro in Tokyo; Editing by Bernadette Baum