US National Security Adviser Michael Flynn has resigned after facing criticism over whether he discussed the possibility of lifting US sanctions on Russia before Donald Trump took office.
Retired General Keith Kellogg, who has been the White House National Security Council chief of staff, was named acting national security adviser.
Donald Trump will make the decision as to who should be appointed to the vacated position.
Kellogg, retired General David Petraeus, who is a former CIA director, and Robert Harward, who is a former deputy commander of US Central Command, are under consideration for the position, a White House official said.
Will Denselow speaks to TRT World from New York.
Vulnerable to blackmail
Flynn's resignation came after it was reported that the justice department warned the White House weeks ago that Flynn could be vulnerable to blackmail because of contact with Russian officials before Trump took power on January 20.
Such interaction could potentially be in violation of a law banning private citizens from engaging in foreign policy, known as the Logan Act.
In his formal resignation letter, Flynn acknowledged in the period leading up to Trump's inauguration, "I inadvertently briefed the vice president-elect and others with incomplete information regarding my phone calls with the Russian ambassador."
"I have sincerely apologized to the president and the vice president, and they have accepted my apology."
Breaking: text of Flynn's resignation letter pic.twitter.com/KGue1cJFzL— Zeke Miller (@ZekeJMiller) February 14, 2017
Flynn had initially told Trump advisers he did not discuss sanctions with the Russian envoy during the transition but later told White House officials that he may have discussed sanctions with the ambassador.
Congressional Democrats expressed alarm at the developments surrounding Flynn and called for a classified briefing by administration officials to explain what had happened.
"We are communicating this request to the Department of Justice and FBI this evening," said Democratic representatives John Conyers of Michigan and Elijah Cummings of Maryland.