NEW YORK (AP) -- His administration dealt a significant blow after less than a month in office, President Donald Trump must now fill a vital national security post after the resignation of embattled national security adviser Michael Flynn.
Flynn stepped down late Monday, ending days of speculation about his fate following reports that he had misled Vice President Mike Pence and other officials about his discussions with Russia prior to Trump's Jan. 20 inauguration.
Whoever emerges as Trump's choice will lead the National Security Council at a time when the young administration is grappling with a series of national security challenges, including North Korea's reported ballistic missile launch.
The White House is also dealing with fallout from the rocky rollout of one of Trump's executive orders on immigration, which has been blocked by the courts. The order was intended to suspend the nation's refugee program and bar citizens of seven Muslim-majority countries from entering the United States.
Trump named retired Lt. Gen. Keith Kellogg as the acting national security adviser, one of three candidates the president has under consideration:
RETIRED LT. GEN. KEITH KELLOGG
Kellogg had previously been appointed the National Security Council chief of staff and, along with Flynn, advised Trump on national security and foreign policy issues during the campaign. He had been considered for national security adviser before the post went to Flynn.
Kellogg was chief operating officer of the Coalition Provisional Authority in Iraq, the interim governing body following the fall of Saddam Hussein in 2003. He previously worked as executive vice president of research and technology for Virginia-based information technology firm CACI International, a contractor for defense, intelligence and homeland security agencies.
The most audacious choice would likely be former CIA director David Petraeus. A retired four-star general, Petraeus was bounced from his position atop the intelligence agency in 2012 after he it was revealed that he had passed classified information to his biographer, who had also become his mistress.
He later pleaded guilty to one misdemeanor charge of mishandling classified information. He was also fined $100,000 and remains on probation.
But during the campaign, Trump spoke sympathetically about Petraeus' plight despite his frequent criticisms of his Democratic opponent Hillary Clinton for mishandling classified materials. Petraeus was briefly under consideration to become secretary of state before Trump picked Exxon CEO Rex Tillerson.
RETIRED VICE ADM. ROBERT HARWARD
Robert Harward, a former Navy SEAL, served as deputy commander of the United States Central Command under Gen. James Mattis, who is now defense secretary. Harward served on the National Security Council under President George W. Bush and commissioned the National Counter Terrorism Center.
Upon retirement in 2013 after a nearly 40-year career in the Navy, Harward became chief executive officer for defense and aerospace giant Lockheed Martin in the United Arab Emirates. Trump has recently been in very public negotiations with Lockheed over the cost of its F-35 fighter jet program.