Ukraine's National Anti-Corruption Agency (NABU) and Specialised Anti-Corruption Prosecutor's Office (SAP) on Friday said it was"conducting investigative actions (searches) at the head of the Office of the President of Ukraine. The investigative actions are sanctioned and are being carried out as part of an investigation," NABU said in a statement.

The NABU did not say what the searches were in connection with.

Yermak is Ukrainian President Volodymyr Zelensky's most important ally, but a divisive figure in Kyiv, where his opponents say he has accumulated power, gate-keeps access to the president and ruthlessly sidelines critical voices.

Investigators earlier this month said they had uncovered a $100-million kickback scheme in the strategic energy sector, triggering widespread public anger at a time when Russia is hammering the country's power grid, causing blackouts and threatening heating outages throughout winter.

Zelensky has put him in charge of negotiations with the United States to rework a 28-point plan proposed by President Donald Trump to end Russia's war in Ukraine, which Kyiv viewed as heavily favouring Moscow.

Yermak said Friday he was cooperating with the investigation.

"There are no obstacles for the investigators. They have been given full access to the apartment, and my lawyers are present on-site, cooperating with the law enforcement officers. From my side, there is full cooperation," he said on social media.