The Kiev home of powerful pro-Russian lawmaker Viktor Medvedchuk was being searched on Tuesday as part of an ongoing investigation into treason, Ukraine's SBU security service told AFP.
Medvedchuk, 66, is a close ally of Russian President Vladimir Putin and has been repeatedly criticised in Ukraine for doing business in Russia, which backs separatists fighting Kiev's army.
Medvedchuk and his wife Oksana Marchenko in February were listed in a criminal probe into the "financing of terrorism".
"We confirm the searches" are being carried out, the SBU said, adding that further details would be issued later.
Prosecutor General Iryna Venedyktova announced that Medvedchuk and another pro-Moscow lawmaker Taras Kozak were confirmed as suspects in the case.
The MPs "are suspected of high treason and attempts to plunder national resources in the Ukrainian Crimea", annexed by Moscow in 2014, Venedyktova wrote on Facebook.
In a statement Medvedchuk's party -- Opposition Platform - For Life -- said the searches and the treason investigation were acts of "revenge and reprisals" by the authorities.
"Such actions of the authorities are an open and cynical political reprisal against the opposition politician," the statement said.
"There is no politics in the actions of the Ukrainian law enforcement system," Venedyktova said Tuesday at a press conference with the SBU chief Ivan Bakanov.
The prosecutor general said that in 2015 Medvedchuk planned to launch the extraction of natural resources off the Black Sea coast belonging to Ukraine and monopolised by Russia after the annexation of Crimea.
Investigators released audio recordings on which voices resembling that of Medvedchuk and Dmitry Kozak, then Russian Deputy Prime Minister, discuss the case.
The lawmakers are also suspected of transmitting information about a secret Ukrainian military unit to the Russian side in 2020, Bakanov said.
Kozak is currently in Russia and doesn't plan to return to Ukraine, while Medvedchuk is in Ukraine, but law enforcement officers are unable to ascertain his location at the moment, Bakanov added.
Ukraine has been fighting separatists backed by Russia in the eastern Donetsk and Lugansk regions since 2014.
The fighting has claimed more than 13,000 lives, according to the United Nations.
Kiev accuses Moscow of sending troops and arms to fuel the conflict, referring to the separatists and those who support them as terrorists.
Ukrainian President Volodymyr Zelensky has banned three pro-Russia television channels formally owned by Kozak, but linked to Medvedchuk.
Zelensky's press service described the channels as a "propaganda tool" and said they receive funding from Russia.