Former Ukrainian prime minister Arseniy Yatsenyuk said Monday he would sue Russia in the European Court of Human Rights over allegations that he fought Kremlin forces in Chechnya.
His spokeswoman Olga Lappo wrote on Facebook that Yatsenyuk intended to prove "the entire absurdity of Russia's justice system".
Yatsenyuk headed the Kiev government from Ukraine's ouster of its Russian-backed leadership in a February 2014 revolution until his resignation over a seeming failure to fight corruption in April 2016.
He was cheered on by Kiev's US and EU allies during the early stages of his premiership for his reformist economic agenda and unbending desire to pull Ukraine out of Russia's orbit.
But the 43-year-old former banker was a bitter hate figure of Russian President Vladimir Putin -- a strongman Yatsenyuk accused of seizing Crimea by force and then pouring troops into eastern Ukraine between March and April of 2014.
The subsequent war in Ukraine's eastern rust belt in which Russia denies involvement has claimed more than 10,000 lives.
The Black Sea peninsula of Crimea remains under Russian control but is globally recognised as part of Ukraine.
Moscow's charges that Yatsenyuk was supposedly fighting alongside Chechen rebels during their 1994-1996 war for independence from Russia first emerged in 2015.
The Russian Investigative Committee -- a powerful counterpart to the US CIA -- accused Yatsenyuk of being involved in at least two military confrontations in 1994-1995.
Yatsenyuk was only 21 at the time and officially studying law at Ukraine's prestigious Chernivtsi National University.
But pro-Kremlin media circulated what Kiev said were doctored photos of a man who somewhat resembled Yatsenyuk posing together with several of Chechnya's most feared warlords of the time.
The first Chechen campaign ended with the republic's de facto independence. The lawless north Caucasus region was finally brought under Russian control during a brief war launched by Putin in October 1999.
Yatsenyuk has laughed off the charges of fighting against Russians alongside the Chechens.
But the story was circulated widely in Russia and resulted in Moscow's demand for his extradition for trial.
Yatsenyuk spokeswoman Lappo said the former Ukrainian cabinet leader would appeal to the European rights court because he could not expect justice in Russia.
A formal lawsuit with the Strasbourg-based court may only be filed after all other avenues in local court systems -- in this case those involving Russia -- are exhausted.
Lappo said Yatsenyuk would turn to the ECHR as soon as Russia rejected his appeal.
"Russia lacks a justice system," Lappo wrote.
"This absurd case will be brought to a close by the European Court of Human Rights."
Yatsenyuk currently has no position in government but heads Ukraine's second-largest parliamentary party.