The US Treasury Secretary has warned that new tariffs on $156 billion worth of Chinese goods will be imposed if a trade deal is not finalised by December 15.
Steven Mnuchin said he still hoped a tentative US-China deal agreed last week to suspend fresh tariffs would be formally approved.
But his caution seemed in contrast with US President Donald Trump's optimism last week.
Wall Street shares dipped at the open on worries about reaching a final deal.
Last Friday, the US agreed to hold off on a planned 25% to 30% tariff increase on Chinese imports that was due to come into effect Tuesday.
It followed two days of talks in Washington between Liu He, China's vice-premier, and US officials.