LONDON/WASHINGTON—U.S. stocks continued their downward trend on Tuesday, opening sharply lower as safe havens gained ground amid investor nerves about tensions between Russia and the West and the prospect of the U.S. Federal Reserve tightening monetary policy soon. All three major U.S. indices opened trading down at least 1 percent, resuming a sharp selloff that had been briefly upended around the close of Monday’s trading. U.S. stocks posted their worst week since 2020 last week. World stocks are on course for their biggest monthly drop since the COVID-19 pandemic hit markets in March 2020. The Dow Jones Industrial Average fell 2.28 percent in early trading, while the S&P 500 lost 2.72 percent and the Nasdaq Composite dropped 2.92 percent. The MSCI world equity index, which tracks shares in 45 nations, was down 1.74 percent. A build-up of Russian troops on Ukraine’s border has triggered fears in the West that Russia …