Moscow was poised on Thursday to retaliate against the United Kingdom's expulsion of nearly two dozen Russian diplomats, after a former Russian spy and his daughter were poisoned on British soil by a Russian nerve agent.

Russian Foreign Minister Sergei Lavrov accused the U.K. of "boorish" behavior, and said that Moscow's response to the diplomatic expulsions would come "very soon," according to Reuters.

Russian state-owned news agency RIA Novosti reported Thursday that Lavrov had previously said that Russia would expel British diplomats from the country in response to the U.K.'s move.

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The back-and-forth between Russia and the U.K. comes as British officials ratchet up pressure on Moscow over what they allege was a Russian government attempt to kill former Russian double agent Sergei Skripal and his daughter, Yulia, using a military-grade nerve agent.

The Skripals remain hospitalized in critical condition. Moscow has denied any involvement in the poisonings.

British Prime Minister Theresa May Theresa Mary MayAre US-Japan relations on the rocks? Trump insulted UK's May, called Germany's Merkel 'stupid' in calls: report Bolton says Boris Johnson is 'playing Trump like a fiddle' MORE moved on Wednesday to expel 23 Russian diplomats believed to have ties to Russian intelligence. She also condemned what she said was Moscow's contemptuous response to the use of a chemical weapon in Europe.

"They have treated the use of a military-grade nerve agent in Europe with sarcasm, contempt and defiance," she said Wednesday.

The decision marked the largest expulsion of Russian diplomats from the U.K. since the Cold War.

Dmitry Peskov, a spokesman for the Kremlin, accused Britain of an unwarranted provocation, and called London's position on the matter "absolutely irresponsible."

"These are all signs of a provocation against our country," he said, according to Reuters. "The position of the British side seems absolutely irresponsible to us."