Russian aluminum giant Rusal saw its share price halved on the Hong Kong stock exchange Monday after Washington placed it under sanctions, putting the metals major at risk of defaulting on part of its debt.
As the Hong Kong stock market closed at 0800 GMT, the company's share price had tumbled 50 percent to HK$2.34 ($0.30, 0.25 euros).
The fall represented a loss of more than 3.5 billion euros ($4.3 billion) for the company, which is headed by billionaire Oleg Deripaska, and which accounts for some seven percent of the world's aluminum production.
The collapse came after U.S. President Donald Trump on Friday announced sanctions against Russian oligarchs close to Vladimir Putin following the diplomatic crisis sparked by the poisoning of former spy Sergei Skripal.
Washington has accused Deripaska of operating for the Russian government. Other magnates hit by sanctions include Alexei Miller, director of state-owned energy giant Gazprom.
Senior U.S. officials described the measures as being aimed at Putin's "inner circle" and said any assets held by the businessmen in areas under U.S. jurisdiction would be frozen.
In a statement, Rusal said the sanctions "may result in technical defaults in relation to certain credit obligations of the group."
More broadly, the latest wave of sanctions also saw the Russian stock market index RTS dive 4.94 percent and the MOEX index plunge 3.94 percent as of 0900 GMT.
Russia's currency also took a hit, with the dollar and euro rising to 58.82 and 72.24 against the rouble respectively.
In all, Trump's administration targeted seven oligarchs, 12 companies they own or control, 17 senior Russian officials and a state-owned arms export company.
The sanctions follow weeks of tit-for-tat diplomatic measures between Russia and Western states following the poisoning of a Russia former double agent in Britain last month.
The U.S. said however that Friday's action was taken under a law passed to punish Russia for its alleged bid to interfere in the 2016 U.S. presidential vote, as well as its intervention in conflicts in Syria and Ukraine.
Moscow has vowed a "tough response" to the sanctions.