A bumper crop of potential candidates for national security adviser is emerging following John Bolton's abrupt exit from the White House on Tuesday.

Amid a number of disagreements with Bolton -- including over recently scrapped negotiations with the Taliban over the future fo Afghanistan -- President Trump announced in a tweet that Bolton’s “services are no longer needed” and said that he would be naming a new national security adviser next week.

TRUMP SAYS JOHN BOLTON IS DEPARTING WHITE HOUSE, SAYS THEY 'DISAGREED STRONGLY'

At least seven people are believed to be in the mix. They include, according to multiple sources: the U.S. ambassador to Germany, Ric Grenell; U.S. Special Representative for Iran Brian Hook; Special Representative for North Korea Stephen Biegun; the ambassador to the Netherlands Pete Hoekstra; and Rob Blair, an aide to White House Chief of Staff Mick Mulvaney. A source close to the discussions said multiple senators called the White House on Grenell’s behalf on Tuesday.

Appearing on Fox News' "Special Report" on Tuesday, South Carolina GOP Sen. Lindsey Graham said Hook, Retired Army Lt. Gen. Keith Kellogg and former deputy national security adviser Rick Waddell are under consideration.

"Those are three names that the president mentioned to me," Graham said. "There are others on the list."

Earlier in the day, the president signaled that he'd fired Bolton – a characterization Bolton himself challenged.

“I informed John Bolton last night that his services are no longer needed at the White House,” Trump tweeted.

He added: “I disagreed strongly with many of his suggestions, as did others in the Administration, and therefore I asked John for his resignation, which was given to me this morning."

While Bolton swiftly challenged Trump's version of events -- saying he first offered to resign -- the two have had well-known disagreements on a range of hot-button national security issues, perhaps most significantly on plans for a troop drawdown in Afghanistan.

Bolton's removal comes after the hawkish adviser was reportedly sidelined from high-level discussions about military involvement in Afghanistan on the heels of opposing diplomatic efforts in the region.

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“Simply put, many of Bolton’s policy priorities did not align with POTUS,” a White House official told Fox News on Tuesday.

While Trump announced a 4,000-troop increase in 2017 as part of an effort to break the stalemate in the country, he has been moving toward agreeing to a phased withdrawal of troops. Some 14,000 U.S. troops have remained in Afghanistan, advising and assisting Afghan forces and conducting counterterrorism operations.

Inside the administration, Bolton also advocated caution on Trump's strategy with North Korea, and was against Trump's decision last year to pull U.S. troops out of Syria. Bolton also led a quiet effort inside the administration and with allies abroad to convince the president to keep U.S. forces in Syria to counter Islamic State and Iranian influence in the region.

Bolton became Trump’s third national security adviser in April 2018, replacing H.R. McMaster, who had himself been named as successor to Michael Flynn.

Fox News’ Gillian Turner, Catherine Herridge Brooke Singman, John Roberts and The Associated Press contributed to this report.