WASHINGTON DC (REUTERS) - President Donald Trump is evaluating US national security adviser Michael Flynn over his Russian contacts, Trump's spokesman said on Monday, pointedly declining to make a public show of support for his embattled aide.

A statement from White House press secretary Sean Spicer, read to reporters crowded around his office, left Flynn's status in doubt, an hour after Trump aide Kellyanne Conway said the president had full confidence in Flynn.

Flynn had told Vice President Mike Pence he had not discussed US sanctions against Russia with Russian officials in the weeks before Trump took office Jan 20, prompting Pence to defend him in subsequent television interviews.

In recent days Flynn has acknowledged he might have discussed sanctions with the Russians but could not remember with 100 per cent certainty, which officials said had upset Pence, who felt he had been misled.

Officials said Flynn has apologized to Pence twice, including in person on Friday.

"The president is evaluating the situation. He is speaking to Vice President Pence relative to the conversation the vice president had with General Flynn and also speaking to various other people about what he considers the single most important subject there is, our national security," Spicer said.

Flynn was an early supporter of Trump and shares his interest in shaking up the establishment in Washington.

The White House statement, arranged during a meeting between Trump, White House chief of staff Reince Priebus and Spicer, suggested that the review into Flynn's activities stretched beyond the conversations he had with Russian officials.

Some news reports have focused on accusations that there has been dysfunction in the operation of the National Security Council with Flynn at the helm.