Michael Flynn, the White House national security adviser, privately discussed sanctions against Russia with Moscow's ambassador to the United States in the month before President Donald Trump took office, according to reports.

Senior US officials interpreted the contacts as a "potentially illegal" signal to Russia that it could expect a reprieve from sanctions imposed by the administration of President Barack Obama, the Washington Post reported.

It had previously been established that Mr Flynn, former Director of the US Defense Intelligence Agency, exchanged phone calls and text messages with Sergei Kislyak, the Russian ambassador, and the FBI is examining the contacts.

There were five calls on Dec 29, the day Mr Obama expelled three dozen Russian diplomats in retaliation for hacking of the US presidential election.

Mr Flynn initially denied he had discussed sanctions with the Russian ambassador, and the Trump administration said the purpose of the communications was to set up a call between the incoming US president and Russian leader Vladimir Putin.

But nine senior officials told the Washington Post sanctions were discussed.

A spokesman for Mr Flynn has now indicated that "while he had no recollection of discussing sanctions, he [Mr Flynn] couldn't be certain that the topic never came up".