Ukraine's National Anti-Corruption Agency (NABU) and Specialised Anti-Corruption Prosecutor's Office (SAP) on Friday said they were"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.

NABU did not say what the searches were in connection with. The raids are another blow to Zelensky, who is facing a mounting Russian offensive in the east just as the United States, a crucial ally, tabled a surprise plan to end the war that heavily favoured Moscow.

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 as 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 US 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.