The Tokyo Stock Exchange's Nikkei 225 index was down 1.5 percent when the traders went to lunch, with the broader Topix index pointing to a minus 1 percent.
In Hong Kong, the Hang Seng was down 2.4 percent at the same time. The Shanghai Stock Exchange was closed due to a holiday.
The tensions between the world's two largest economies increased over the weekend, after the US accused China of having broken an agreement from May on tariffs.
China "firmly rejects" the allegations and states that Washington "has made false accusations" that "seriously contradict the facts”.
Last week, US President Donald Trump claimed that China "completely broke the agreement with us", without elaborating on how the agreement had been broken.
