Russia will expel British diplomats stationed within the country in a retaliatory move for British Prime Minister Theresa May announcing she would expel 23 Russians from her country following an apparent assassination attempt on a former Russian intelligence officer living in the United Kingdom.

Russian Foreign Minister Sergei Lavrov told Reuters that Moscow would take reciprocal action to expel British diplomats from the country. "We will, of course," he reportedly said.

May in a Wednesday address to Parliament said that Russia was behind the poisoning of former Russian spy Sergei Skripal and his daughter earlier this month. The two are still undergoing treatment to recover from the deadly Russian nerve agent.

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The attack marks the first known use of the substance that was developed by the Soviet military since World War II and has marked a new low in relations between Britain and Russia in the post-Cold War era.

Russia has denied the accusations and has accused Britain of fabricating the story.

May said the expelled Russians, whom she accused of being Kremlin spies, would have one week to leave the country and that all planned high-level diplomatic meetings between the two countries would be suspended immediately.

The prime minister demanded that Russia give the British government an official response by Tuesday at midnight.

The United Nations Security Council will discuss the attack in an emergency session on Friday after key allies of the U.K., including the U.S., have condemned the poisoning as an overt attack by the Russians on British soil.