Newly announced USA sanctions-and the potential for a second round of actions in 90 days-roiled Russia's currency and blue-chip stocks as the country braced for further economic pain amid uncertainties over the Trump administration's commitment to enforcement.
Medvedev said that while the USA says that the sanctions are meant to punish Russia's "bad" behavior, their real goal is to sideline a rival.
"And we'll have to respond to it economically, politically or in any other way, if need be".
Skripal and his daughter were poisoned by the Novichok military-grade nerve agent in the English city of Salisbury in March.
This comes one day after Washington announced it would impose more sanctions on Russia over the United Kingdom nerve agent attack that targeted a former Russian double agent and his daughter.
A second set of measures could be substantially broader and would be imposed if Moscow fails to meet a 90-day deadline to provide "reliable assurances" it will no longer use chemical weapons, allow on-site inspections by the United Nations or other global observer groups, and respond to other USA demands.
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Russian President Vladimir Putin has previously denied allegations that Russia was behind the poisonings, saying in March that it was "unthinkable that we would do such a thing".
Russian Prime Minister Dmitry Medvedev says that Moscow would consider it a "declaration of economic war" and would retaliate "economically, politically, or, if needed, by other means" if the United States imposes bans on Russian banks or their use of a particular currency.
A terse release from the State Department said that the United States had determined Russian responsibility for the attack in Salisbury, England - a British conclusion the administration had already accepted - under a 1991 U.S. law on biological and chemical weapons use that requires the president to impose sanctions.
That could include the USA withdrawing support for global loans and US bank loans, blocking Russian airlines from landing in the USA, and suspending diplomatic relations.
-Russian relations. A 1991 law, however, mandates sanctions once the president makes a determination that a country is responsible for a chemical or biological weapons attack. Trump made no public comments on the matter, and communications aides referred all questions to the State Department, which said the USA still sought improved ties with Moscow. And unlike the coordinated trans-Atlantic sanctions enacted in response to the Ukraine crisis in 2014, America's European allies - who together trade far more with Russian Federation that the United States does - aren't following suit this time with new sanctions of their own. He said the new sanctions amount to "inflicting a punishment in the absence of a crime, in the tradition of lynch law".
If Russia does not meet three criteria - ending its use of such weapons, assuring the US that it has no plans to use them again, and offering to let worldwide observers verify that - then there will be a second, harsher round of sanctions in 90 days.