Washington imposed sanctions on Friday on seven Russian oligarchs, including Rusal's former president Oleg Deripaska, 12 companies they own or control, as well as 17 senior Russian government officials. The company floated its shares in London in November, raising $1bn (£710m), despite concerns that it could face sanctions related to Deripaska's ties to Putin.
The Kremlin slammed the sanctions.
The US actions target Russian oligarchs whose companies have wide-ranging involvement in worldwide capital markets. Other magnates hit by sanctions include Alexei Miller, director of state-owned energy giant Gazprom.
En+ Group and Rusal both released statements on Monday that said "it is highly likely" that the sanctions would "be materially adverse" to their businesses.
The US Treasury said the sanctions were linked to Russia's actions in Crimea, Syria and Ukraine and its interference in western democracies.
In Moscow the dollar-denominated RTS index ended 11.2% lower and the rouble-based MOEX Russian index shed 7.7%, while the rouble fell 3% against the United States dollar.
Evraz, the steel company controlled by the Chelsea football club owner Roman Abramovich, was the biggest faller in the FTSE 100, dropping nearly 15%.
"The US list may not be the final list and it feels like there will be more sanctions, so investors do not know which, if any, stocks to hold", said John Meyer, mining analyst at SP Angel.
A Hong Kong-based equity sales director said today the stock prices are under pressure because the sanctions require investors subject to United States jurisdiction to ditch the stocks within a month.
Under the sanctions, Americans are generally prohibited from doing business with the companies, and people of other nationalities could face sanctions themselves if they facilitate transactions for the firms.
Russian prime minister Dmitry Medvedev said on Monday that he would instruct the government to develop initiatives to support companies affected by the sanctions and also consider retaliatory measures in response.