The US actions target Russian oligarchs whose companies have wide-ranging involvement in worldwide capital markets.
As we reported earlier, the U.S. imposed sanctions against nine Russian companies, and the question on sanctions against 12 companies was specified.
The stock fell tumbled 49 per cent to HK$2.38 at around 0710 GMT, according to pricing from the Hong Kong Exchanges & Clearing website.
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In Moscow, the other exchange where its shares trade, they were down 23 percent in mid-session business. This reportedly cost its largest shareholder Vladimir Potanin around $1.3 billion, even though he was not among the 26 people and 15 companies blacklisted on April 6 by the USA authorities in response to the diplomatic fallout of the Sergei Skripal poisoning. Other magnates hit by sanctions include the director of state-owned energy giant Gazprom, Alexei Miller. "The situation is pretty egregious from the point of view of legality, it tramples on all kinds of norms, and so a thorough analysis is necessary", he said.
Rusal assesses that the sanctions may result in technical defaults in relation to certain credit obligations of the group, and the company is evaluating the impact of such technical defaults on its financial position.
Moscow said it was looking to support Russian companies hit by the sanctions.
The Russian stock market also fell heavily.
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EN+, which only listed in London in November having raised $1 billion from City investors despite fears over the potential of further sanctions being imposed on Russian Federation, said it would focus on maintaining its operations and ensuring it "abides by all applicable laws and regulations".
The sanctions follow weeks of tit-for-tat diplomatic measures between Russian Federation and Western states following the poisoning of a Russian Federation former double agent in Britain last month.
"This is due to sanctions imposed by the United States on Russian oligarchs and their companies on Friday", it added.
The health of Sergei Skripal, the former Russian spy who was found slumped on a bench with his daughter in the English city of Salisbury, is now improving.
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The US Treasury said the sanctions were linked to Russia's actions in Crimea, Syria and Ukraine and its interference in western democracies.