Imposing a deadline on a negotiation for a deal you desperately want is a risky tactic if your only move is to walk away from a deal you desperately want.

"This statement is very harmful to the Peace Negotiations with Russia," Trump said in a social media post. He said Crimea was lost years ago "and is not even a point of discussion." Zelenskiy on Tuesday reiterated that Ukraine will not recognize Russia's occupation of the Crimea.

Ukraine is insisting on a full ceasefire in order to negotiate with Russia, as U.S. officials — hoping for a quick solution — backed out of London talks.