Three of Ben Carson's top campaign aides resign

Republican Presidential candidate Ben Carson is interviewed prior to the Republican Presidential Debate, in Las Vegas, Nevada December 15, 2015.

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Last Updated Dec 31, 2015 5:00 PM EST

Ben Carson's campaign manager Barry Bennett and communications director Doug Watts stepped down from their posts with the presidential campaign Thursday morning, "effective immediately."

"We respect the candidate and we have enjoyed helping him go from far back in the field to top tier status," Watts said of himself and Bennett in a statement Thursday. "We are proud of our efforts for Dr Carson and we wish him and his campaign the best of luck."

The resignations, reported first by the Des Moines Register early Thursday, come just 31 days before the Iowa caucuses.

Of the decision to leave the campaign, Bennett told the Washington Post, "My frustration level has peaked with Dr. Carson's outside advisers."

Bennett also told the Associated Press that Carson's longtime business manager, Armstrong Williams, is the adviser who has Carson's ear, even though Williams does not have a formal role in the campaign.

Carson is "one of the smartest men I've ever worked for," Bennett said, but added that he believes Carson has become Williams' "script reader."

"You have to surround yourself with good people," Bennett said. "And he hasn't demonstrated that he can do that. No one wants Armstrong Williams anywhere near the Oval Office."

Williams replied Thursday: "Barry and I agree. I will be nowhere near the Oval Office when Dr. Carson is elected president. I will remain in my private practice."

Williams also disputed Bennett's characterization that his influence is inappropriate, and said the departures were more firings than resignations. "I'm sure Barry resigned because he wanted total control and he wasn't going to have that," Williams said.

Shortly after Bennett and Watts tendered their resignation, a third aide also announced she would also quit the campaign.

According to the Des Moines Register, deputy campaign manager Lisa Coen resigned Thursday afternoon:

In a statement, the Carson camp announced retired army general Bob Dees as campaign chairman and Ed Brookover as campaign manager.

Bennett and Watts' decision to leave the campaign came a week after Carson told The Associated Press in an interview that he was considering a major staff shakeup, only to walk back those comments hours later, declaring that he had "full confidence" in his team.

Williams arranged for that interview without Bennett's knowledge. Carson's subsequent statement of support for his team was issued after discussing his initial comments with Bennett and Watts, but Bennett said Thursday that those events were evidence his place in the campaign had become untenable.

Carson "told everybody else 'nobody wants staff changes,'" Bennett recalled. "Why the hell did you say it then? Armstrong had given him the talking points."

The interview "was Armstrong's calculation against us," Bennett said. "Ben was just the script reader. It was horribly embarrassing to us, the whole campaign staff. One hundred fifty people went home for Christmas with their families wondering whether they would keep their jobs. Excellent timing."

Bennett described Carson as "surprised" by the resignations. Williams, who says he spoke with Carson after the candidate spoke with Bennett, described Carson as "calm, confident, reassured and ready to move forward."

"This allows Dr. Carson a fresh start," Williams said.

Williams said he spoke with Dees, the new campaign chairman, on Thursday and described their relationship as "wonderful."

"I've spoken with the good general, congratulated him," Williams said. "We've been with Dr. Carson since the beginning of this operation."

Taylor said the campaign turnover was not unexpected and that Carson is actively engaged with the decision making. It helps that Carson's Iowa campaign director, Ryan Rhodes, will remain in his position and perhaps take on greater responsibilities, Taylor said.

"We've been moving forward in Iowa the whole time," Taylor said.

CBS News' Erica Brown contributed to this report.