Washington: US President Donald Trump expressed displeasure Monday over Iran's appointment of Mojtaba Khamenei as its new supreme leader, calling it "a big mistake."

According to Anadolu Agency, Trump stated, "I don't know if it's going to last ... I think they made a mistake," during an interview with NBC News. He had earlier conveyed to the New York Post that he was "not happy" with the choice, emphasizing that Khamenei would "have to get approval from us."

Mojtaba Khamenei, 56, assumed the role of Iran's third supreme leader since the 1979 revolution on Sunday. He succeeds his father, Ayatollah Ali Khamenei, who was killed in a US/Israeli airstrike on his Tehran compound on February 28, after holding the position for 37 years.

Trump did not dismiss the possibility of seizing Iranian oil but deemed it premature to discuss the matter. "Certainly people have talked about it," he remarked, referencing Venezuela as a precedent, where the US secured over 80 million barrels of oil following the capture of Venezuelan leader Nicolas Maduro earlier this year. "It's too soon to talk about that," he added, regarding Iran.