President Donald Trump's national security adviser, Michael Flynn, is struggling to get past a controversy over his contacts with Russian officials before Trump took office, conversations that officials say have raised concerns within the White House.

Top White House officials have been reviewing Flynn's contacts with the Russians and whether he discussed the possibility of lifting US sanctions on Russia once Trump took office, which could potentially be in violation of a law banning private citizens from engaging in foreign policy.

Flynn is a retired US army general and former director of the Defense Intelligence Agency. An early supporter of Trump, he has been a leading advocate to improve US relations with Russia.

Flynn had initially denied discussing sanctions with the Russians in the weeks before Trump took office on January 20 and Vice President Mike Pence went before the television cameras to repeat the denial and defend Flynn.

When a Washington Post report emerged last week quoting officials saying the subject of sanctions had in fact come up, Flynn left open the possibility that he had discussed sanctions but could not remember with 100 per cent certainty, an administration official said.

A second administration official stressed that Pence made his comments based on a conversation with Flynn. Pence is said to be troubled by the possibility of being misled.

Flynn has apologised to Pence and others over the incident, the first official said.

A third official said the uproar prompted White House chief of staff Reince Priebus to review the matter with other top officials as Trump played host to Japanese Prime Minister Shinzo Abe over the weekend in Florida.

The White House did not respond to a request for comment.

Trump has yet to weigh in on the subject, promising to reporters on Friday that he would look into it.