At 01:37 GMT, the Shanghai Composite index was down 54.53 points or 1.73 percent at 3,098.23, while China's blue-chip CSI300 index was down 1.63 percent at 3,841.28.
Chinese H-shares listed in Hong Kong fell 0.82 percent to 12,028.75, while the Hang Seng Index was down 0.77 percent at 30,075.97.
The trade war fears led to the largest fall China's main stock indexes in six weeks on Friday.
Vice Premier Liu He told US Treasury Secretary Steven Mnuchin on Saturday that the United States had flouted trade rules with an inquiry into intellectual property and China would defend its interests, China's state media reported.
Despite threats of retaliation from China over US plans to impose tariffs on up to $60 billion in Chinese goods, Treasury Secretary Steve Mnuchin on Sunday said President Donald Trump had no intention of backing down and was not worried about a trade war.
Tech firms were relatively stable on Monday morning, with the tech-heavy start-up board ChiNexpt up around 0.5 percent.