Russian Prime Minister Dmitry Medvedev lashed new US sanctions as a "full-fledged economic war" on Moscow, saying they crushed hopes for repairing ties and demonstrated President Donald Trump's "total weakness ... in the most humiliating way."
Trump reluctantly signed off on the new sanctions Wednesday, bowing to domestic pressure after the White House failed to scupper the bill or water it down.
Medvedev warned the move would have "consequences", saying "it ends hopes for improving our relations with the new US administration."
"Second, it is a declaration of a full-fledged economic war on Russia," Medvedev said on his Facebook page.
Taunting the notoriously thin-skinned US president, Medvedev added: "The Trump administration has shown its total weakness by handing over executive power to Congress in the most humiliating way".
Trump signed the legislation behind closed doors and his reluctance was on full display in an angry signing statement in which he called the legislation "significantly flawed."
"In its haste to pass this legislation, the Congress included a number of clearly unconstitutional provisions," he said, including curbs on the president's ability to "negotiate" with Russia.
"I built a truly great company worth many billions of dollars. That is a big part of the reason I was elected. As president, I can make far better deals with foreign countries than Congress," Trump claimed.
The legislation -- which also includes measures against North Korea and Iran -- targets the Russian energy sector, giving Washington the ability to impose sanctions on companies involved in developing Russian pipelines, and placing curbs on some Russian weapons exporters.
Iran, too, reacted angrily, saying the new sanctions against it "violated" its 2015 nuclear deal with world powers, and warned it would respond "appropriately."
With regards to Russia, the sanctions notably constrains Trump's ability to waive the penalties -- a statement of mistrust from the Republican-controlled Congress, which remains unsettled by Trump's warm words for President Vladimir Putin.
In his statement, Trump said: "The Framers of our Contitution put foreign affairs in the hands of the President. This bill will prove the wisdom of that choice."
In a searing rebuttal, maverick Republican Senator John McCain said: "The framers of our constitution made the Congress and the President coequal branches of government. This bill has already proven the wisdom of that choice."
"I hope the president will be as vocal about Russia's aggressive behaviour as he was about his concerns with this legislation."
- Russia meddling -
The sanctions seek to penalise the Kremlin for allegedly meddling in the 2016 US presidential election and for Russia's annexation of Crimea.
Trump said he would "honour" some of the bill's provisions, but stopped short of saying it would be fully implemented.
The White House said only that Trump would give Congress's "preferences" mere "careful and respectful consideration."
"I am signing this bill for the sake of national unity. It represents the will of the American people to see Russia take steps to improve relations with the United States," Trump said.
Trump had received the legislation at 1:53 pm on Friday, but waited until Wednesday to sign it.
The delay had raised speculation that Trump might veto or try to somehow shelve the sanctions, which were approved in a 98-2 Senate vote.
By signing it, he avoided the humiliating prospect of Congress overriding his veto.
Expecting the signature, Moscow preemptively ordered Washington to reduce its diplomatic presence in Russia to 455 people before September 1 -- bringing it in line with the size of Russia's mission in the United States.
The Kremlin said that Trump signing the sanctions "doesn't change anything," in a less adversarial statement than those issued by Medvedev and the foreign ministry.
The foreign ministry said the sanctions against Russia had put global stability at risk, calling them a "dangerous" and "short-sighted" policy.
"We have already shown that we are not going to leave hostile acts unanswered... and we obviously reserve the right to take retaliatory measures," it said.
US Secretary of State Rex Tillerson said he will meet with his Russian counterpart Foreign Minister Sergei Lavrov over the weekend, but warned US-Russia ties could still get worse.
Tillerson said the US Congress's decision to pass the sanctions bill had made attempts to thaw ties "more difficult."
A special prosecutor is investigating whether Trump advisers colluded with what US intelligence has concluded was an attempt by Russia to covertly support the real estate mogul's 2016 campaign.
The US president, who often called for warmer ties with Moscow during the White House race, has furiously denied the charge.