NBC News reported that the senator called the New York businessman "a small insecure money-grubber who fights for no one but himself" and a "thin-skinned bully driven by insecurity and hate."
Later, Trump responded in a phone conversation with NBC, saying that he hopes Warren is tapped as Clinton's vice-presidential running mate because she's "easy" to compete against.
"She is one of the least productive senators in the United States Senate. We call her Pocahontas for a reason," Trump said, reiterating his earlier tweet.
He also said to NBC that the controversial nickname is "a great insult to Pocahontas."
Trump reiterated his doubts regarding Warren's Native American heritage, saying that she hasn't been able to prove it and has used it to advance her career.
"Elizabeth Warren is a total fraud. ... It's a racist thing that she's done when she created a phony heritage," he said.
The Republican presidential candidate has previously expressed similar doubts about other people that claim Native American ancestry. He previously said that some of the people who run casinos on reservations "don't look like Indians" to him and "don't look like Indians to Indians," during a 1993 testimony before Congress.
Warren's office did not immediately respond to CNBC's request for comment.
— NBC News' Hallie Jackson, Charlie Gile and Alex Seitz-Wald contributed to this report.