LOS ANGELES -- US President Donald Trump said Sunday that Ukrainian President Volodymyr Zelensky is attempting to back out of a rare earth minerals deal with the United States, warning that if he does that, he could have "problems."

"I think Zelensky, by the way, he's trying to back out of the rare earth deal, and if he does that, he's got some problems -- big, big problems," Trump told reporters on Air Force One.

Trump said that a deal had already been made regarding rare earth minerals, but now, Zelensky is looking to renegotiate. "He wants to be a member of NATO. Well, he was never going to be a member of NATO. He understands that, so, if he's looking to renegotiate the deal, he's got big problems," said Trump.

On March 20, Trump announced that a rare earth minerals deal with Ukraine would be signed "very shortly." The agreement, initially scheduled for signing in Washington at the end of February to grant the US access to Ukraine's mineral resources, was delayed following a tense exchange of words between the two leaders at the White House.

Zelensky said last Friday that Ukraine has received a new draft version of the minerals deal proposed by Washington, while its text was "entirely different" from an earlier framework agreement that he had been set to sign with Trump.