"So Ukraine now has got its own Quisling. Sitting on foreign soil begging a foreign army to give his country to him," Bildt wrote on Twitter Tuesday.
Vidkun Quisling was a Norwegian politician who backed Nazi Germany and was executed in October 1945. His name has since come to symbolise a national traitor collaborating with an occupation force.
Bildt admitted to news agency TT that his choice of words was not very diplomatic, but, he said, "you should call a spade a spade".
"This is a man without a political future and if you go to another country and say 'I want a foreign army to conquer my country', then there is word from that which springs from European history," Bildt said.
On Monday, the EU condemned Russia's "acts of aggression", calling on Moscow to recall its troops to their bases in Crimea. US President Barack Obama said Russia is on "the wrong side of history" and faces growing isolation for violating Ukraine's sovereignty.