President Donald Trump knew weeks ago there were problems with Michael Flynn’s Russia phone calls, a White House spokesman has said.
The president was told by the Justice Department at the end of January that Mr Flynn may have misled US officials and the public, said Sean Spicer.
He was asked to quit three weeks later due to an “erosion of trust”.
Republicans have joined congressional calls for an investigation into Mr Flynn’s contacts with Russia.
Mr Flynn was also reportedly questioned by FBI agents in his first days as national security adviser, according to US media.
Mr Flynn resigned over allegations he discussed US sanctions with a Russian envoy before Donald Trump took office.
The retired army lieutenant-general initially denied having discussed sanctions with Ambassador Sergei Kislyak, and Vice-President Mike Pence publicly denied the allegations on his behalf.