"I think it's time for Ukraine to get a new president," Trump told reporters at the White House on Saturday.
"Zelenskiy could have settled this war many times, many many times, and he chooses not to," Trump claimed.
"I suggest they get a new leader who can make a deal, because he could have made many deals that, for some reason, he never does," Trump said.
Russian President Vladimir Putin has repeatedly rejected proposals for an unconditional ceasefire, while US-led diplomatic initiatives have also failed to produce a breakthrough.
Trump also accused Zelenskiy of contributing to global diesel shortages by ordering strikes on Russian oil refineries.
Trump, who on Friday struck a deal with Putin to buy diesel from Russia in an attempt to address high fuel prices ahead of the US midterm elections, has been urging Zelenskiy to stop strikes on Russian energy infrastructure.
He has also blamed record US diesel prices on Ukraine rather than the US-led war in Iran, which has elevated world energy prices.
"He wants to make problems for the world. He wants to make problems for our farmers, our ranchers, with diesel fuel," Trump said, before departing for a rally in Tennessee.
Washington plans to lift sanctions on Russian diesel to allow the deliveries to proceed. The Kremlin confirmed the agreement, which drew swift criticism from Zelenskiy.
The US president said he had warned Zelenskiy against targeting Russian refineries.
"There's plenty of targets. Don't hit refineries," Trump said, recounting his warning to the Ukrainian leader.
"He better damn well stop."
Ukraine has repeatedly targeted Russian energy infrastructure as part of its efforts to weaken Moscow's ability to finance its war.
Ukrainian and European officials reacted to Trump's diesel deal with dismay, saying it would help pay for Russia's war efforts.
Trump also suggested that if he had been US president in 2022, "that war would have never started. There was no reason that war with Ukraine and Russia started. It started due to incompetence".
Trump's extraordinary intervention came after Russian strikes killed at least 20 people in Ukraine's Zaporizhzhia region, Ukrainian officials said on Saturday.
Three children were among those killed when Russian aerial bombs hit residential buildings in Zaporizhzhia, Ukraine's State Emergency Service said Saturday. At least 18 people were wounded.
Russia and Ukraine are locked in a long-range aerial war while their armies remain largely deadlocked on the front line, more than four-and-a-half years since Moscow's full-scale invasion.
Ukraine is struggling to counter escalating Russian bombardments that have killed more than 60 people over the past four days.
"Today, Russia expressed its thanks for the lifting of sanctions on diesel exports by dropping five glide bombs on residential buildings in Zaporizhzhia," Zelenskiy wrote Saturday on X.
He said every additional billion dollars Russia earns from exports would help pay for weapons components, drone upgrades, missile production and the recruitment of mercenaries, and that Moscow would only be ready to stop when its ability to fight diminished.
Zelenskiy said Ukraine was co-ordinating steps to counter the extra revenue Russia would receive, and that he had spoken to French President Emmanuel Macron about shoring up air defences.
In Kyiv, people gathered at a memorial to fallen soldiers on Independence Square greeted the deal with a mix of anger and resignation.
with dpa
