US President Donald Trump called Ukrainian leader Volodymyr Zelensky a "dictator" on Wednesday, widening a personal rift with major implications for efforts to end the conflict triggered by Russia's military offensive three years ago.

The United States has provided essential funding and arms to Ukraine, but Trump made an abrupt policy shift by opening talks with Moscow just weeks after he returned to the White House.

"A dictator without elections, Zelensky better move fast or he is not going to have a country left," Trump wrote on his Truth Social platform.

Zelensky was elected in 2019 for a five-year term and has remained leader under martial law imposed as his country fights for its survival.

Trump savaged Zelensky, saying "he refuses to have elections, is very low in Ukrainian polls, and the only thing he was good at was playing (Joe) Biden 'like a fiddle.'"

"In the meantime, we are successfully negotiating an end to the War with Russia, something all admit only 'TRUMP,' and the Trump Administration, can do."

Zelensky's popularity has fallen, but the percentage of Ukrainians who trust him has never dipped below 50 percent since the conflict started, according to the Kyiv International Institute of Sociology.

Under Biden, the United States had lauded Zelensky as a hero and hammered Moscow with sanctions as Ukraine battled against advancing Russian soldiers.

But Trump held a press conference on Tuesday in which he tore into the Ukrainian leader and repeated Kremlin narratives such as the claim that Ukraine started the war.

Trump's invective drew shock from Europe where German Chancellor Olaf Scholz said it was "wrong and dangerous" to call Zelensky a dictator.

In Washington, Trump's former vice president Mike Pence also issued a stinging rebuke.

"Mr President, Ukraine did not 'start' this war. Russia launched an unprovoked and brutal invasion claiming hundreds of thousands of lives," Pence wrote on X. (AFP)