His administration dealt a significant blow after not even 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.

Mr Flynn stepped down late on Monday, ending days of speculation about his fate following reports that he had misled Vice President Mike Pence and other officials about his contacts with Russia.

Mr Flynn conceded that discussions of sanctions may have come up during several calls with the Russian ambassador during the transition period leading up to Mr Trump's January 20 inauguration.