President Donald Trump revived one of his favorite taunts against U.S. Sen. Elizabeth Warren on Monday, referring to the Massachusetts senator as "Pocahontas" at an event honoring three Navajo Code Talkers.

"You were here long before any of us were here," Trump said. "Although we have a representative in Congress who they say was here a long time ago. They call her Pocahontas."

In the past, Native American leaders have called Trump’s attacks on Warren offensive and distasteful. Some Democrats have called the remark racist.

"This was supposed to be an event to honor heroes, people who put it all on the line for our country. And people who because of their incredible work saved the lives of countless Americans and our allies," Warren said in responding to the comments on MSNBC. "It is deeply unfortunate that the president of the United States cannot even make it through a ceremony honoring these heroes without having to throw out a racial slur."

She said that Trump "does this over and over" thinking he’s going to "shut me up with this. It hasn’t worked in the past. It is not going to work in the future."

The president has spent months feuding with Warren, an outspoken Wall Street critic who leveled blistering attacks on Trump during the campaign. He seized in particular on questions about her heritage, which surfaced during her 2012 Senate race challenging incumbent Sen. Scott Brown.

During that campaign, law school directories from the Association of American Law Schools from 1986 to 1995 surfaced that put Warren on the association’s list of “minority law teachers” when she was teaching at the University of Texas and the University of Pennsylvania. Warren said she listed herself with Native American heritage because she hoped to meet people with similar roots.

In a 2012 interview with The Associated Press, Warren said she and her brothers were told of the family’s heritage by their parents, the late Don and Pauline Herring.

Brown pressed Warren to release more information about how she described her heritage to potential employers. Warren said she never sought proof of ancestry because she didn’t think it was necessary.

White House press secretary Sarah Huckabee Sanders dismissed criticism about Trump’s comments by saying she didn’t believe the president had used a racial slur.

“What most people find offensive is Sen. Warren lying about her heritage,” she said.

The Navajo Nation suggested Trump's remark Monday was an example of "cultural insensitivity" and resolved to stay out of the "ongoing feud between the senator and President Trump."

"All tribal nations still battle insensitive references to our people. The prejudice that Native American people face is an unfortunate historical legacy," Navajo Nation President Russell Begaye said in a statement. He added that the Navajo Nation remains honored by the White House recognition of the code talkers.