At least half a dozen contenders are under consideration to replace John Bolton as Donald Trump's national security advisor.
The search for Mr Trump's fourth national security adviser in less than three years began after he announced in a tweet on Tuesday that Mr Bolton's services were "no longer needed."
Over the following 24 hours several of Mr Bolton's allies also departed.
The changes leave Mike Pompeo, the secretary of state, with increased influence over US foreign policy. He and Mr Bolton frequently disagreed.
Lindsey Graham, the Republican senator and friend of Mr Trump, said he had spoken to the president about a successor.
Those being considered included retired Army general Keith Kellogg, currently national security adviser to vice president Mike Pence.
Brian Hook, the US special representative for Iran, and Rick Waddell, a former deputy national security adviser, were also possibilities.
Mr Graham said: "Those are three names the president mentioned to me. There are others on the list.
"Keith Kellogg’s a retired general, competent, capable, confident, has the president’s trust, I like him a lot."
The others on the list were believed to include Richard Grenell, the US ambassador to Germany, Pete Hoekstra, the US ambassador to the Netherlands, and Stephen Biegun, the special representative for North Korea.
A group of Republican senators were believed to be lobbying the president to pick Mr Grenell.
Charles Kupperman, Mr Bolton's deputy and a former defence company executive, has taken over as acing national security adviser.
Mr Trump is expected to make his choice known next week.
White House officials indicated Mr Bolton's departure after 17 months was because his views had not "aligned" with Mr Trump's on a variety of issues.
There were also suggestions that others within the administration blamed Mr Bolton for a media report that Mr Pence was opposed to pursuing a Taliban peace deal.
The report annoyed Mr Trump. who declared it "fake news". Mr Bolton denied he was responsible.
Mr Bolton was preceded as national security adviser by H.R. McMaster, and Michael Flynn.
Mr Trump, speaking at the White House, called Mr Bolton "Mr Tough Guy" and criticised him over the Iraq war.
The president said: "Mr Tough Guy. Now, we're in for seven trillion dollars in the Middle East. He was very out there in wanting to have them do it. I thought it was a terrible mistake."
Mr Trump said he got along "very well" with Mr Bolton personally, but he "made some very big mistakes."
That included Mr Bolton's suggestion of a "Libyan model" for North Korea, which "set us back" with North Korean negotiations, and was "not smart".
The president said: "What a disaster. I don't blame Kim Jong-un for what he said after that. He wanted nothing to do with John."
He said Mr Bolton was also "way out of line" on Venezuela, and added: "John wasn't getting along with people in the administration who are very important. I wish John the best."