According to the CNN TV channel, O'Brien was previously expected to let go of approximately a dozen officials once the impeachment trial of US President Donald Trump was over. The official reason is to streamline the work of the council, which has grown from 100 people to 236 during the Obama administration.
On Friday, the White House fired NSC adviser Lieutenant Colonel Alexander Vindman, who had been a witness in the impeachment inquiry.
On Wednesday, the Republican-majority US Senate voted 52-48 to acquit Trump on impeachment charges for abuse of power and obstruction to Congress.
On 18 December, Democrats in the House of Representatives impeached Trump claiming that he had made efforts to enlist Ukraine’s help in investigating allegations of corrupt business practices by his 2020 presidential rival Joe Biden's son Hunter in exchange for military aid to Kiev.
Trump has repeatedly said he has broken no laws and characterized the impeachment as a hoax and a political witch hunt by Democrats to reverse the results of the 2016 presidential election.