BANGKOK—World shares were mixed on Friday as investors watched developments in the Ukraine after the U.S. warned of a high risk of a Russian invasion. Benchmarks rose in London, Paris, and Shanghai but fell in Hong Kong and Tokyo. Oil prices fell and U.S. futures were higher. The U.S. has issued some of its starkest, most detailed warnings yet about how a Russian invasion of Ukraine might unfold, and its Western allies went on high alert for any attempts by the Kremlin to create a false pretext for a new war in Europe. Russia is thought to have some 150,000 military forces near Ukraine’s borders and NATO allies have questioned Moscow’s assertions it was pulling back troops from exercises that had fueled fears of an attack. The crisis has hung over markets for weeks, raising volatility. Russia is a major energy producer and if it were to invade Ukraine, other …