Explained: Why Trump's National Security Adviser Is in Hot Water Over Russia Call

U.S. National Security Adviser Michael Flynn is on thin ice over his communication with a Russian diplomat. Flynn, a retired U.S. army general, has long called for closer cooperation with Moscow. However, he might have cooperated a bit too closely, too soon.

Top White House officials told Reuters they spent the weekend reviewing Flynn's contacts with the Russians made before Donald Trump took office, and before he, Flynn, had any official government job.

In particular, a call he made to Russia's ambassador to the U.S. where he allegedly discussed new sanctions imposed by the Obama administration.

That call may have broken U.S. laws, and Flynn may have lied about it afterward.

It's illegal for a private citizen to engage in foreign policy. Flynn initially denied he discussed sanctions on the call, and U.S. Vice President Mike Pence publicly vouched for Flynn's integrity.

One problem: American spies are in the habit of listening into calls made to foreign diplomats, and the transcript of that call may tell a different story.

Perhaps Flynn, who used to run the Defense Intelligence Agency, should have known this.

The Washington Post reporting Flynn had, in fact, discussed sanctions. When pressed, Flynn backed off his earlier denial, saying he could not remember with one hundred percent certainty whether it had come up.

Questions still looming over the current U.S. administration's ties to Moscow after a collection of U.S. intelligence agencies concluded Russia interfered in the U.S. election in an effort to swing the contest toward Donald Trump.

Suspicions further raised after unverified reports Russia may have compiled a dossier of compromising information about Donald Trump, which it could employ as blackmail.

The U.S. president expected to face tough questions about Flynn's conduct at a news conference Monday.