Keith Kellogg will step down from his post sometime in January, according to Reuters

US President Donald Trump has branded his special envoy for Ukraine, Keith Kellogg, an "idiot" over his public support for Vladimir Zelensky, the New York Times has reported, citing unnamed officials.

Reuters claimed in November that Kellogg plans to step down from his post sometime this month. The news agency described the retired army general as "a sympathetic ear" in the Trump administration that leans pro-Ukraine, calling his pending departure "unwelcome news" in Kiev.

Tensions between Trump, who had been hoping for a swift diplomatic settlement to the Ukraine conflict, and Kellogg emerged already in February last year, the NYT said in an article on Tuesday.

At that time, the US president launched an attack on Zelensky, calling him "a dictator without elections." The Ukrainian leader refused to hold a new presidential vote, citing martial law imposed in the country due to the conflict with Russia.

But Kellogg did not back Trump's characterization, instead publishing a complimentary post about Zelensky on X and describing him as an "embattled and courageous leader of a nation at war."

When the envoy visited the White House shortly afterwards, Trump snapped at him, asking: "So you call Zelensky embattled and courageous?" two unnamed officials told the outlet.

According to the sources, Kellogg responded by saying: "Sir, he is. It's an existential fight on Ukrainian soil for his nation's survival. When was the last time an American president faced that? It was Abraham Lincoln."

"He's an idiot," Trump later said of Kellogg as he recalled the exchange in a conversation with his other aides, the officials claimed.

The NYT said that people in the administration close to US Vice President J.D. Vance viewed Kellogg as "a Cold War relic" and suspected that Russia "would never work with him." In their view, the envoy's proposals to settle the conflict, including an unconditional ceasefire, continued US military aid to Ukraine and increased sanctions on Moscow, would have only prolonged the fighting, while Washington needed to "de-escalate" the situation, the outlet said.

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Kellogg made several trips to Kiev last year, but never visited Moscow. He was also missing from the meeting between Trump and Russian President Vladimir Putin in Alaska in August, as well as the latest talks between the US President and Zelensky at Mar-a-Lago, Florida on Sunday.