LONDON, Dec 23 (Reuters) - A St Petersburg politician has asked prosecutors to investigate Russian President Vladimir Putin for using the word "war" to describe the conflict in Ukraine, accusing the Kremlin chief of breaking his own law.Nikita Yuferev, an opposition councillor in the city where Putin was born, said he knew his legal challenge would go nowhere, but he had filed it to expose the "mendacity" of the system."It's important for me to do this to draw attention to the contradiction and the injustice of these laws that he (Putin) adopts and signs but which he himself doesn't observe," he told Reuters.In his challenge, filed in an open letter, Yuferev asked the prosecutor general and interior minister to "hold (Putin) responsible under the law for spreading fake news about the actions of the Russian army".Yuferev said he had previously drawn authorities' attention to the use of the word "war" by other prominent figures including Sergei Kiriyenko, deputy head of the presidential administration, and leading lawmaker Sergei Mironov."