Zelensky's ongoing diplomatic tour of Europe follows intensive talks between US and Ukrainian negotiators over the weekend which failed to produce a deal to which Kyiv could agree.
On Sunday, Trump indicated that he viewed Zelensky as the main obstacle to securing a peace deal, something he has made a key foreign policy goal and which the president claimed he would be able to achieve rapidly during the 2024 presidential election campaign.
He told reporters that Russia was "fine" with the peace plan outlined to both sides by the US, but that he was a "little disappointed that Zelensky hasn't read it".
Almost simultaneously, Zelensky said he was waiting to briefed by his chief negotiator Rustem Umerov following three days of discussions with his US counterparts in Miami.
"Some issues can only be discussed in person," said Zelensky, ahead of a meeting in London attended by UK Prime Minister Sir Keir Starmer, French President Emmanuel Macron and German Chancellor Friedrich Merz.
The summit was widely viewed as a show of support for Ukraine as it seeks to resist White House pressure.
Trump called Europe's leaders "weak" in an interview with Politico published on Tuesday.
On whether Europe could help end Russia's war with Ukraine, the US president said: "They talk, but they don't produce, and the war just keeps going on and on."
No 10 said there had been an agreement that the US-led talks represented a "critical moment" to ramp up support for Ukraine, and repeated calls for a "just and lasting peace... which includes robust security guarantees".
But it is not yet clear what form that would take.