WASHINGTON — New sanctions against Russia will be imposed later this month for illegally using a chemical weapon in an attempted assassination of a former spy and his daughter in Britain earlier this year, the United States said.

The penalties come despite President Donald Trump’s efforts to improve relations with Russia and its leader, Vladimir Putin, and his harsh criticism of the investigation into Russian interference in the 2016 election.

Putin’s spokesman Dmitry Peskov said the U.S. move runs contrary to a “constructive” atmosphere at the Trump-Putin summit last month, and he strongly denied any Russian role in the poisoning in Britain.

“In our view, these and earlier restrictions are absolutely unlawful and don’t conform to international law,” Peskov said.

A Russia Foreign Ministry spokeswoman said Thursday the restrictions represent a new attempt to “demonize Russia.” Spokeswoman Maria Zakharova said Moscow will work out retaliatory measures, adding that “ultimatums against Russia are useless.”

The State Department said Wednesday the U.S. made the determination this week that Russia had used the Novichok nerve agent to poison Sergei Skripal and his daughter, Yulia, and that sanctions would follow. It said Congress is being notified of the Aug. 6 determination and that the sanctions would take effect on or around Aug. 22, when the finding is to be published in the Federal Register.

Those sanctions will include the presumed denial of export licenses for Russia to purchase many items with national security implications, according to a senior State Department official who briefed reporters on condition of anonymity as he was not authorized to do so by name.

The U.S. made a similar determination in February when it found that North Korea used a chemical weapon to assassinate North Korean leader Kim Jong Un’s half-brother at the airport in Kuala Lumpur, Malaysia, in 2017.

Skripal and his daughter were poisoned by the Novichok military-grade nerve agent in the English city of Salisbury in March. Both eventually recovered. Britain has accused Russia of being behind the attack, which the Kremlin vehemently denies.

Months later, two residents of a nearby town with no ties to Russia were also poisoned by the deadly toxin. Police believe the couple accidentally found a bottle containing Novichok. One of them died.

The U.S. had joined Britain in condemning Russia for the Skripal poisoning and joined with European nations in expelling Russian diplomats in response, but it had yet to make the formal determination that the Russian government had “used chemical or biological weapons in violation of international law or has used lethal chemical or biological weapons against its own nationals.”

British Prime Minister Theresa May welcomed the U.S. decision. Her Downing Street office issued a statement saying the move sends “an unequivocal message to Russia that its provocative, reckless behavior will not go unchallenged.”

Peskov insisted that “there can’t be any talk about Russia having any relation to the use of chemical weapons,” adding that Britain has failed to present any evidence to back the claim and stonewalled Russia’s proposal for a joint probe.

The Russian Embassy in Washington said the “draconian” new sanctions against Russia weren’t backed by any facts or evidence, noting that while the U.S. said it has enough information to conclude that Russia is to blame, it refused to disclose what it has, saying the information is classified.

Konstantin Kosachev, the chairman of the foreign affairs committee in the upper house of the Russian parliament, said Thursday that the U.S. has behaved like a “police state, threatening and torturing a suspect to get evidence.” He added that the new sanctions amount to “inflicting a punishment in the absence of a crime in the tradition of lynch law.”

Leonid Slutsky, the head of the lower house’s foreign affairs committee, denounced the sanctions as a manifestation of “unbridled Russophobia” and mockery of international law, saying that Russia may respond with countersanctions.

Russia has not said how it might retaliate. Peskov said in a conference call with reporters that it needs to see what specific action the U.S. takes before retaliating. He insisted that Russia’s financial system is strong enough to withstand shocks from the new penalties.

In the morning, the Russian ruble sank to the lowest levels since April on the news of the new sanctions before recovering slightly later in the day. The shares of Russian state-controlled banks, the national carrier Aeroflot and other companies also tanked.

Several members of Congress had expressed concern that the Trump administration was dragging its feet on the determination and had missed a deadline to publish its findings.

Lawmakers praised Wednesday’s announcement.

“The administration is rightly acting to uphold international bans on the use of chemical weapons,” said Ed Royce, R-Calif., chairman of the House Foreign Affairs Committee. Royce had previously accused Trump of ignoring the Russian nerve agent attack.

“The mandatory sanctions that follow this determination are key to increasing pressure on Russia. Vladimir Putin must know that we will not tolerate his deadly acts, or his ongoing attacks on our democratic process,” Royce said Wednesday.

While criticized as too keen to strike up a friendship with Putin, Trump maintains that he’s been tough on Moscow. His administration has sanctioned a number of Russian officials and oligarchs for human rights abuses and election meddling.

In March, the Trump administration ordered 60 Russian diplomats — all of whom it said were spies — to leave the United States and closed down Russia’s consulate in Seattle in response to the Skripal case. The U.S. said at the time it was the largest expulsion of Russian spies in American history.

The State Department announced a number of possible exceptions to the new sanctions. Waivers have been issued for foreign assistance and space flight activities, while commercial passenger aviation and other commercial goods for civilian use will be assessed on a case-by-case basis, according to the official who briefed reporters.

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