Russia said on Monday several of its diplomats had been expelled from Poland and that a number of Polish diplomats had left Russia after Moscow took "adequate" measures in return, Reuters reported. Ties between Russia and European Union member states have been strained by the crisis in Ukraine and by EU sanctions imposed after Russia annexed the Crimea peninsula from Ukraine. Poland has been one of Moscow's most outspoken critics. The Russian Foreign Ministry did not say why the Russian diplomats had been expelled and the Polish embassy declined immediate comment. Diplomatic sources said four Polish diplomats left the country on Sunday after being given 48 hours to leave. "The Polish authorities have taken an unfriendly and unfounded step," Russia's Foreign Ministry said in a statement. "In connection with that, Russia has undertaken adequate measures in response and a number of Polish diplomats have already left the territory of our country for activities incompatible with their status." A spokesman for Polish Foreign Minister Grzegorz Schetyna said on his Twitter microblog: "We are closing the case."