WASHINGTON — President Donald Trump, frustrated by his staff's handling of the abuse allegations against Rob Porter, is increasingly venting about Chief of Staff John Kelly and speculating about potential replacements, according to multiple sources familiar with the matter.

Kelly said Friday evening that he had not offered to resign, although a source acknowledged that he is always willing to leave if that's what the president wants.

One senior administration official and three other people briefed on the president's conversations told NBC News that Trump is angry at Kelly's initial statement of effusive support about Porter's character — and then the quick walk back the next day.

After Porter's two ex-wives made allegations of physical and verbal abuse in The Daily Mail, Kelly said in a statement on Tuesday: "Rob Porter is a man of true integrity and honor and I can't say enough good things about him."

On Wednesday, Kelly issued a statement saying that "there is no place for domestic violence in our society," but that he stood by his original comments. Porter, who has denied the abuse allegations, left his job as White House staff secretary on Thursday.

Kelly said Friday that he was first notified in November on the security clearance process involving Porter. But Kelly added that he did not know more until Tuesday when "learned it was true. Forty minutes later he was gone."

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At the White House on Friday, Trump spoke favorably about Porter.

"We wish him well," the president told reporters. "He worked very hard. Found out about it recently and I was surprised by it. ... Obviously, tough time for him. He did a very good job when he was at the White House."

The president added: "We hope he has a wonderful career. ... Very sad when we heard about it. He's very sad. He also, as you probably know, he says he's innocent."

The president is also frustrated about Kelly's recent comments that have created headlines — including his statement last month that Trump's view on immigration was "evolving" and his widely criticized remark this week that some "Dreamers" were "too lazy" to sign up for DACA.

Two of the sources said the brewing dissatisfaction has Trump openly musing about potential replacements for the chief of staff job. Inside the West Wing, there's rampant speculation about who those replacements might be.

Among the names being circulated by Trump for chief of staff is current Office of Management and Budget Director Mick Mulvaney, who also heads the Consumer Financial Protection Bureau.

Kelly is said by a source to feel "irritated" and annoyed he's at the center of coverage. He feels frustrated that even though he's answered questions about Porter, he still has to keep dealing with the fallout, according to the source.

One of the sources pointed out that the Porter incident could give Trump a specific reason to cite if he chooses to remove Kelly, but all four sources caution that it is more likely Kelly stays on the job.

"Clearly, the president is frustrated with the status quo," a source close to the White House said when asked about the fallout from the Porter scandal. Even before that happened, Trump chafed at being isolated under Kelly's strict management structure and attempt to control the flow of information to the president.

"You can get away with (that) for a couple of months," but at some point Trump will "rebel," the source added.

People close to the president have said they witnessed him frequently venting about his advisers, yet doesn’t always pull the trigger on getting rid of them. And the president is leery of another staff shake-up at this time. One source also noted that Kelly is viewed as the guy "doing all the work."

One White House staffer said that despite the president's frustration, "I don’t think he wants to fire anybody because there's too much drama."

Trump's unhappiness isn't just with Kelly. Two sources told NBC that the president is also frustrated over Communications Director Hope Hicks' handling of the Porter controversy.

That Trump is frustrated with Hicks is unusual. She has long enjoyed the president's favor and benefited from her loyalty to him.