(CNN) Democratic Sen. Kirsten Gillibrand of New York told CNN on Monday that President Donald Trump should resign over allegations of sexual assault.

"President Trump has committed assault, according to these women, and those are very credible allegations of misconduct and criminal activity, and he should be fully investigated and he should resign," Gillibrand told CNN's Christiane Amanpour in an exclusive interview.

"These allegations are credible; they are numerous, " said Gillibrand, a leading voice in Congress for combating sexual assault in the military. "I've heard these women's testimony, and many of them are heartbreaking."

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If he does not "immediately resign," she said, Congress "should have appropriate investigations of his behavior and hold him accountable."

Later on Monday, Democratic Sen. Ron Wyden of Oregon also called on Trump to resign in a tweet, encouraging Capitol Hill to investigate.

"These women are right. If @realDonaldTrump won't resign, Congress must investigate allegations by many, many women that he sexually assaulted and harassed them. No one is above the law," he tweeted.

Two other Democratic senators -- Cory Booker of New Jersey and Jeff Merkley of Oregon -- called for Trump's resignation over the weekend.

Responding to UN Ambassador Nikki Haley saying Sunday that Trump's accusers "should be heard," Gillibrand said Monday, "Not only should women be heard, but they should be believed."

White House press secretary Sarah Sanders responded to Haley's comments at the news briefing Monday, where she reiterated that Trump "thinks it's a good thing that women are coming forward" but noted that he denies the allegations.

"As the President said himself, he thinks it's a good thing that women are coming forward but he also feels strongly that a mere allegation shouldn't determine the course," Sanders said. "And in this case, the President has denied any of these allegations, as have eyewitnesses ... several reports have shown those eyewitnesses also back up the President's claim in this process, and again the American people knew this and voted for the President and we feel like we're ready to move forward in this process."

This story has been updated and will continue to update with additional developments.