Downing Street rejected Steve Witkoff's characterization of Vladimir Putin as a "humanitarian" following the US negotiator's visit to the Russian president in St Petersburg on Wednesday.
The British government said Putin "has killed thousands of people, both innocent Ukrainians and his own people," according to a No 10 statement.
Witkoff, who serves as Donald Trump's chief negotiator on the Ukraine war, met with Putin as part of efforts to broker an end to the invasion. The US has been pressuring Kyiv to agree to a settlement.
The dispute over language reflects deeper tensions over how Western officials describe the Russian leader and his conduct during the nearly three-year conflict. Witkoff's visit and subsequent characterization signal Trump's administration may be pursuing settlement negotiations that differ substantially from the British government's public position on accountability for Russian actions.