National security adviser John Bolton has informed National Security Council employees that staff changes are imminent, including an influx of new people and a reorganization, a Trump administration source told Fox News on Wednesday.

In a memo sent by Bolton on Wednesday morning, he informed staffers that the changes could include combining higher-level director positions.

The source said that while some at the National Security Council (NSC) welcome Bolton's changes, the news will upset many White House staffers who are overworked and overwhelmed as a result of the administration's mercurial foreign policy. The source added that it is possible that skilled NSC staffers will leave their posts as a result of the changes, making it hard to attract other potential staffers.

Bolton is the third national security adviser to serve under President Donald Trump in the 15 months of the Trump administration. He succeeded H.R. McMaster earlier this month. And McMaster succeeded Michael Flynn, who resigned in February 2017. (Flynn has since pleaded guilty to making false statements to the FBI in the course of the bureau's investigation into contacts between Russian officials and Trump's presidential campaign.)

A number of top national security officials -- including NSC spokesman Michael Anton, homeland security adviser Tom Bossert and deputy national security advisers Ricky Waddell and Nadia Schadlow -- have either been pushed out by Bolton or have chosen to leave because of his appointment. One of those former NSC officials, retired Army Lt. Gen. Keith Kellogg, was chosen Monday by Vice President Mike Pence to serve as his national security adviser.

Also Wednesday, Bolton went to the Pentagon for breakfast with Defense Secretary Jim Mattis, the first in a planned series of regular meetings, according to Pentagon spokeswoman Dana W. White. She said the Pentagon hoped Mike Pompeo, Trump's choice for secretary of state, would start attending if he's confirmed.

Fox News' Lucas Tomlinson and The Associated Press contributed to this report.