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"We will act according to the principle of reciprocity and will do the same the Lithuanian side did," Russia's ambassador to Lithuania, Alexander Udaltsov, told reporters.
The ambassador described Lithuania's decision to expel three Russian diplomats as "absolutely unreasonable, absolutely thoughtless" and made in the absence of any proof of them working for intelligence.
Udaltsov gave no details as to when and how many diplomats will be expelled from Russia.
Lithuania is ordering the Russian diplomats out of the country in solidarity with Britain over the poisoning of a former Russian spy.
The ambassador said that Lithuania is expelling two high-ranking diplomats and the defense attaché.