While Prime Minister Shinzo Abe and U.S. President Donald Trump neared an agreement on a U.S.-Japan trade deal over the weekend, it wasn't enough to quell market anxiety over the U.S.-China trade war Monday, when key Tokyo stock indexes plummeted and the yen at one point rose to its highest level against the dollar this year.

The 225-issue Nikkei average, which plunged more than 500 points at one point, ended down 449.87 points, or 2.17 percent, at 20,261.04 on Monday. The Topix, which covers all first-section issues on the Tokyo Stock Exchange, dropped 24.22 points to 1,478.03.

The yen, seen as safe in times of volatility, sharply appreciated, even breaking the ¥105 threshold to hit ¥104.50 against the dollar early Monday morning. The currency was trading at ¥105.73 against the dollar as of 5 p.m. in Tokyo trading.