ST. PETERSBURG, Russia -- A plane belonging to the state airline Aeroflot was being searched Friday by police at a London airport, Russia's Foreign Ministry said. Spokeswoman Maria Zakharova said British authorities did not state a reason for the search, which she called "the latest provocation."

Tensions between London and Moscow are high amid the dispute over the nerve-agent poisoning of a former Russian double agent and his daughter. Britain blames Russia for the poisoning.

Each country has expelled 23 of the other's diplomats. Russia further escalated its response on Friday, ordering Britain to reduce the number of its diplomats in Moscow to the level that Russia has in London. That number wasn't immediately clear.

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Zakharova did not specify the London airport where the search was taking place, but a Heathrow-to-Moscow flight online had its expected arrival delayed by about three hours.