KYIV - Ukraine unveiled a criminal case on Friday against the boss of Russia's Wagner mercenary company, and promised to track down and prosecute the company's fighters who try to flee abroad.

Wagner, run by businessman Yevgeny Prigozhin, an ally of President Vladimir Putin, has recruited thousands of fighters, including convicts from Russian prisons, to wage war in Ukraine. Prigozhin says his group is pivotal in recent battles in the east of Ukraine, among the bloodiest of the war.

"The Prosecutor General's Office has served a notice of suspicion to the head of the private military company 'Wagner'," Prosecutor General Andriy Kostin said in a statement on Facebook that did not identify Prigozhin by name.

"The head of this group is directly responsible for thousands of war crimes. He openly admits his role in the war against Ukraine and, with the Kremlin's permission, resolves staff issues by recruiting tens of thousands of prisoners."

There was no immediate comment from Prigozhin. Last month he dismissed U.S. allegations against Wagner, and the Kremlin said Washington had been smearing Wagner without reason for years.

Under Ukraine's criminal code, suspects in criminal cases must be informed in a "notice of suspicion". The statement did not say how such a notice could have been served to Prigozhin.

Kostin listed accusations against Wagner including encroachment on Ukraine's territorial integrity and waging an aggressive war. He said Wagner mercenaries of all ranks would be held responsible, including those who fled abroad.

Ukraine would send Oslo a "request for investigative actions" against a former Wagner commander arrested in Norway last month, Kostin said.

"I am certain together with international partners we will ensure comprehensive responsibility both for every criminal who came to our land with weapons in his hands and also for their patrons who do business in blood," Kostin said.

(Reporting by Olena Harmash Editing by Timothy Heritage and Peter Graff)