Volodymyr Zelensky has gone out of his way since becoming Ukrainian president to avoid provoking Moscow. But one action has enraged the Kremlin: Kyiv’s swoop against Viktor Medvedchuk, a pro-Russian Ukrainian oligarch and politician who made Russian president Vladimir Putin godfather to his daughter.

In February, the Ukrainian government placed Medvedchuk on its sanctions list, froze his assets and shut down three television channels under his control for allegedly spreading misinformation.

Zelensky’s move against the businessman, an expression of his frustration at a deadlocked peace process that has failed to end the war in eastern Ukraine, may have been a factor behind Russia’s sabre-rattling and massive troop mobilisation along Ukraine’s borders last month, analysts said.

Undeterred, Ukrainian prosecutors last week sought Medvedchuk’s arrest on charges of treason. On Friday, Putin hit back saying Moscow would respond “promptly and properly, bearing in mind all the threats posed to us”. Ukraine, he added, “is being turned slowly but surely into some antipode of Russia, some anti-Russia”.

Zelensky has come under pressure from the Biden administration to clamp down on Ukraine’s oligarchs and step up the fight against corruption.

Medvedchuk, 66, a former presidential chief of staff in the early 2000s, made his fortune as a lawyer before branching into banking and energy. He was used by Zelensky’s predecessors as a back-channel to Moscow and played a role in the Minsk talks that were intended to end the war between Ukrainian and Russian-backed forces in the Donbas region.

But more recently he has been regarded by Ukraine’s pro-western leadership as the voice of the Kremlin, relaying its views through TV channels owned by an associate and via his leadership of Opposition Platform — For Life, the largest of Ukraine’s pro-Russian opposition parties, which opposes the country’s future integration into Nato and the EU.

“Medvedchuk is not only an oligarch but is also the primary agent of influence for the Kremlin in Ukraine for decades,” an adviser to Zelensky said, explaining the crackdown as a pressing part of a broader effort to break oligarch capture of the country’s politics and economy.

“This makes him very dangerous for Ukraine’s national security,” the adviser added.

The businessman was not immediately available for comment.

Medvedchuk has echoed Moscow’s demands for ending the Donbas conflict, urging Ukraine to hold direct talks with the Russian-backed leaders of breakaway regions and to grant them autonomy that would potentially hand them veto power over future EU and Nato integration.

“He is Putin’s asset inside Ukraine,” said Orysia Lutsevych, head of the Ukraine Forum at Chatham House in London. He is “like a magnet for all pro-Russian groups, for pro-Russian sentiments”.

Before they were taken off air, the TV stations under his control had for years aired narratives “almost the same as Russian state TV channels”, according to Natalia Ligachova, editor of Detector Media, a Kyiv-based media watchdog.

“Putin is presented as a hero, Russia is a friendly nation with which we need to have friendly ties, our enemy is the west, Ukrainians are fascists and . . . this is not a real nation after all.”

Through its talking heads or through long monologues by Medvedchuk himself, the channels had spread the view “that Russia is not attacking Ukraine, that Ukraine attacked itself and is itself to blame, that this is a civil war”, Ligachova said.

Medvedchuk has repeatedly denied owning channels 112, NewsOne and Zik. They belonged to Taras Kozak, an associate of Medvedchuk and fellow pro-Russian MP who was also last week charged with treason. Ukrainian authorities said he is in Russia. Medvedchuk said Kozak is in Belarus.

Addressing a court on Thursday which ordered his house arrest, Medvedchuk — who has been sanctioned by the US since 2014 for undermining Ukraine’s independence and territorial integrity — denied wrongdoing and described the charges as “a politically motivated accusation”.

Zelensky’s administration and independent experts have pointed to evidence that the Kremlin in effect financed the broadcasting and political activities of Medvedchuk and his associates by granting them lucrative energy business interests in Russia.

The case is still being investigated, but Ukraine’s general prosecutor and state security chief said Medvedchuk and Kozak were for now being charged with treason related to two separate episodes. These include allegedly passing on to Russia information about a secret military unit, and engaging with Russian officials in offshore hydrocarbon transactions off the coast of occupied Crimea.

“Zelensky’s fight against Medvedchuk is a fight against Putin,” said Hanna Hopko, chair of Ants, a Kyiv-based national interests advocacy group.

The oligarch has meanwhile used his political position to undermine Zelenksy’s reform drive. The president has been struggling to overhaul a notoriously corrupt judicial system many of whose judges were appointed under Viktor Yanukovich, the former pro-Russian president who fled to Russia after being ousted by the 2014 pro-democracy revolution.

In a bid to drive a wedge between Kyiv and its western backers and derail a $5bn IMF rescue, MPs allied with Medvedchuk last year applied successfully to Ukraine’s unreformed constitutional court to neuter recently established anti-corruption institutions. Fearing the court could next cancel farmland sales and other reforms that are conditions for continued western support, Zelensky sidelined its chief justice as authorities launched probes against him.

Mykhailo Pogrebinsky, a political analyst who describes himself as Medvedchuk’s “friend”, said Zelensky was behaving like a “crook” by silencing critical media and trying to squash the leader of a party backed by 3.5m voters in the 2019 election.

But the rise in the president’s poll ratings in recent months suggested that “stamping out these Russian proxies in Ukraine is widely supported in Ukrainian society”, Lutsevych said.

“Its amazing that after the invasion of Crimea and the invasion of the east, that Medvedchuk still managed to operate for so long and strengthen his influence,” she added.