Ukrainian authorities placed former president Petro Poroshenko under formal investigation for high treason on Monday, accusing him of links to financing separatist forces in the eastern Donbass region.
Officials said the accusations against Mr Poroshenko were related to similar charges against pro-Russian politician Viktor Medvedchuk, who has been under house arrest for about six months.
Mr Medvedchuk's and Mr Poroshenko's political parties denied wrongdoing.
Oleksandr Turchynov, a senior official in Mr Poroshenko's European Solidarity party, said the accusation was fabricated on the instruction of President Volodymyr Zelenskiy and would "turn into a farce just like all the previous ones".
Ukraine has been at war with Russian-backed separatists in the Donbass region since 2014.
Mr Medvedchuk, whose political party is the second largest in Parliament, is a Ukrainian citizen but has close ties with Russian President Vladimir Putin, who he says is godfather to his daughter.
Ukraine's state investigative bureau said Mr Poroshenko was suspected of "facilitating the activities" of terrorist organisations in a preliminary conspiracy with an unidentified group of people, including some top officials in Russia.
In October, law-enforcement officials accused Mr Medvedchuk of colluding with officials during Mr Poroshenko's administration to buy coal from mines in separatist-held areas in 2014-2015 as a way of financing the separatists.
Mr Poroshenko's party in October called those accusations a smokescreen to divert attention from the government's own wrongdoing.
Officials said last week that the bureau could not summon Mr Poroshenko for questioning in person. Mr Poroshenko's party said the former president had left the country for a planned trip.
Ukraine faced an acute fuel shortage after separatists seized territory that included coalmines.
Prosecutors accused Mr Medvedchuk of colluding with state officials to block coal purchases from the international market.