President Donald Trump's first choice for national security adviser, General Michael Flynn, resigned late Monday night. Lt. General Joseph Keith Kellogg Jr. has been chosen as the acting NSA while a permanent replacement is found.

In a statement making the announcement, the White House said: "President Donald J. Trump has named Lt. General Joseph Keith Kellogg, Jr. (Ret) as Acting National Security Advisor following the resignation of Lt. General Michael Flynn (Ret)."

Kellogg served in the U.S. Army from 1967 to 2003, and went overseas twice during the Vietnam War, where he earned the Silver Star, the Bronze Star with ā€œVā€ device, and the Air Medal with ā€œVā€ device. He was commander of the 82nd Airborne Division from 1997 to 1998. Before retirement, Kellogg headed the Command, Control, Communications, and Computers Directorate under the Joint Chiefs of Staff.

In his resignation letter, Flynn defended his being in contact with the Russian ambassador.

"In the course of my duties as the incoming National Security Advisor, I held numerous phone calls with foreign counterparts, ministers and ambassadors. These calls were to facilitate a smooth transition and begin to build necessary relationships between the President, his advisors and foreign leaders. Such calls are standard practice in any transition of this magnitude. Unfortunately, because of the fast pace of events, I inadvertently briefed the Vice President Elect and others with incomplete information regarding my phone calls with the Russian Ambassador."

Flynn lasted less than a month in his role.

Three people are being considered as possible successors to Flynn, including Kellogg, David Petraeus and Bob Harward. Petraeus was due to meet Trump at the White House on Tuesday, earlier reports claimed. He headed the CIA for a little over a year, before resigning in 2012 for passing classified information to a lover.