US equity futures turned lower after Trump's comments, and the VIX volatility index - known as the "fear gauge" - rose to its highest level since July 11.
Asian markets were dented by a slide in the Chinese yuan to its lowest for more than a year, after the Chinese central bank set a weaker fixing for the currency for the seventh straight session.
The yuan has shed 7.6 percent of its value against the dollar since the end of the first quarter, hurt by concerns over the China-US trade war and a slowing Chinese economy.
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