BRUSSELS (AP) — The European Union and Britain imposed sanctions Thursday on six Russians, some among the highest-ranked officials in the nation, and a state research institute over the nerve agent poisoning of Russian opposition leader Alexei Navalny.

The move comes a day after Russia's foreign minister threatened the 27-nation EU with retaliatory action. The sanctions had been agreed on by EU foreign ministers on Monday, without names given.

“Russia is our neighbor and shares this continent with us, but we will not give up our principles and convictions when it comes to chemical weapons,” French President Emmanuel Marcon told reporters at a summit of EU leaders in Brussels. He said the bloc must continue “a transparent but demanding dialogue” with Moscow.

Foreign Minister Heiko Maas of Germany, which holds the EU's rotating presidency, said that “only with a clear position and by sticking to principles can we as the European Union make progress with respect to Russia.”

Those hit by the sanctions, which consist of an asset freeze and travel bans in Europe, include Alexander Bortnikov, the chief of Russia’s Federal Security Service, the top KGB successor agency that is in charge of domestic security, and Sergei Kiriyenko, President Vladimir Putin’s deputy chief of staff.

The State Scientific Research Institute for Organic Chemistry and Technology was also targeted. The EU said that institute, which was responsible for destroying Soviet-era chemical weapon, was years ago involved in the development and production of chemical weapons, including the nerve agent Novichok allegedly used to poison Navalny.

The U.K. said it would also apply the EU sanctions and will continue them once it leaves a post-Brexit transition period at the end of the year.

“Any use of chemical weapons by the Russian state violates international law. We are determined to hold those responsible to account,” British Foreign Secretary Dominic Raab said.

Navalny, an anti-corruption investigator who is Putin's most visible political opponent, fell ill on Aug. 20 during a domestic flight in Russia. He was flown to Germany for treatment two days later and is still recovering there.

Last week, tests conducted at labs designated by the Organization for the Prohibition of Chemical Weapons confirmed that Navalny was poisoned by a Novichok nerve agent. They confirmed results found earlier in labs in Germany and elsewhere.

Russian officials have repeatedly denied any involvement in the poisoning and Russian doctors who first examined Navalny have said they found no signs of a poisoning.

Kremlin spokesman Dmitry Peskov called the move “a deliberate unfriendly step towards Russia” and said that the EU “inflicted damage” on the bloc’s relations with Russia.

“Moscow will analyze the situation and will act in accordance with its own interests,” Peskov said, adding that “no logic can be seen in such a decision” by the EU.

EU foreign ministers agreed Monday to impose the sanctions under sanctions to combat the use and spread of chemical weapons, following a push by France and Germany. The legal procedures were completed Thursday.

French Foreign Minister Jean-Yves Le Drian said the EU had acted “with exceptional speed, in keeping with the seriousness of this act and the methods used.”

“This demonstrates a European Union that acts in the face of the new, unacceptable and destabilizing use of a chemical weapon,” he said at a meeting in Paris.

Russian Foreign Minister Sergey Lavrov has suggested that Moscow might retaliate and even sever ties over the Navalny dispute. “The Germans are not planning to provide any facts, despite all international and legal obligations. We respond in kind. This is diplomatic practice," he said Wednesday.

A day earlier, Lavrov had said: “we probably simply have to temporarily stop talking to those people in the West who are responsible for foreign policy and don’t understand the need for mutually respectful dialogue.”

In a phone call Tuesday with Lavrov, EU foreign policy chief Josep Borrell insisted that the EU “wishes to maintain open channels of communication with Russia and to enhance cooperation on issues of mutual interest,” according to a statement from Borrell’s office.

Borrell also underlined that Moscow “must do its utmost to investigate this crime thoroughly in full transparency and to fully cooperate” with the OPCW. He said the EU “will continue to defend its interests and values, including respect for international law and fundamental rights.”

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Daria Litvinova in Moscow, Geir Moulson in Berlin, and Jill Lawless in London contributed to this report.