MOSCOW (AP) — Moscow is awaiting nearly two dozen Russian diplomats ordered to leave Britain as part of a standoff over a nerve agent attack on British soil.

Britain ordered the 23 diplomats to leave by Tuesday, and they're expected in Moscow later, according to Russian media reports.

Russia retaliated by expelling 23 British diplomats, who are expected to leave Moscow in the coming days.

Russia denies involvement in the poisoning of ex-Russian spy Sergei Skripal and his daughter Yulia in the British city of Salisbury earlier this month. They remain in critical condition.

Britain accuses Russia of the poisoning, which Western powers see as an example of increasingly aggressive Russian meddling abroad.

International chemical weapons experts took samples Monday of the nerve agent used, which Britain says is the Soviet-developed Novichok.