According to the State Department, those sanctions will include the presumed denial of export licenses for Russia to purchase many items with national security implications.
Some reports indicated that a possible next wave of sanctions in the fall could target Russia's state-controlled banks and freeze their operations in dollars a move that would deal a heavy blow to the Russian economy.
Medvedev warned the US that such a move would cross a red line.
"If something like a ban on bank operations or currency use follows, it will amount to a declaration of economic war," he said. "And it will warrant a response with economic means, political means and, if necessary, other means. Our American friends should understand that."
Medvedev's tough tone was in stark contrast with past statements by President Vladimir Putin and his lieutenants, who have taken a nonchalant posture while talking about US and other Western sanctions, seeking to downplay their impact on the Russian economy.
The announcement of new US sanctions has rattled the Russian currency and stock markets.
The ruble plummeted to its lowest level since August 2016 in early trading today.
Russia-US ties have sunk to their lowest level since the Cold War times amid tensions over Ukraine, the war in Syria and the allegations of Russian meddling in the 2016 US presidential election.
Medvedev said while the US says that the sanctions are intended to punish Russia's "bad" behavior, their real goal is to sideline a rival.
"It's intended to remove Russia as a strong competitor on the international arena," he said.
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