Russian President Vladimir Putin on Tuesday called for a probe into the poisoning of a former Russian spy in the United Kingdom last month.

Putin made the remarks after meeting with Turkish President Recep Tayyip Erdoğan in Ankara and demanded that Moscow play a role in the probe, according to The Associated Press.

The AP reports that Putin cited the head of the U.K.'s defense laboratory as saying the source of the nerve agent used in the attack had not been identified.

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He went on to say the "speed at which the anti-Russian campaign has been launched causes bewilderment," according to the news agency.

Putin's comments come nearly a month after Sergei Skripal and his daughter, Yulia Skripal, were exposed to a military-grade nerve agent.

U.K. health officials announced last week that Yulia was no longer in critical condition.

Multiple countries, including the U.S., have laid blame on Moscow for the attack, and have expelled Russian diplomats in response.

Russia retaliated last month, announcing that it would expel 60 American diplomats and would shutter the U.S. consulate in St. Petersburg in response to a similar action by Washington.

Russian Foreign Minister Sergey Lavrov also said that Moscow would take action against the two dozen other countries that also banished Russian diplomats.