Former Russian President Dmitry Medvedev warned NATO on Tuesday against providing Ukraine with Patriot missile defence systems and called the alliance a "criminal entity".

"If, as (NATO Secretary-General Jens) Stoltenberg hinted, NATO were to supply the Ukrainian fanatics with Patriot systems along with NATO personnel, they would immediately become a legitimate target of our armed forces," Medvedev wrote on the Telegram messaging app.

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It was not clear from his message whether he was referring to Patriot systems, Ukrainian forces or NATO personnel becoming a target.