The FBI looked into conversations between Mike Flynn, national security adviser to Donald Trump, and Russia's ambassador to the United States in December and found no improper activity.

U.S. officials told the Washington Post that the calls were monitored as part of routine surveillance on Russian officials that had nothing to do with Flynn.

Flynn, a retired three-star general who once sat next to Russian President Vladimir Putin at a dinner celebrating the Kremlin-operated RT news channel that he was paid to attend, was said to have been under investigation for the communications.

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Michael Flynn has come under investigation as part of a counterintelligence examination of communications between Russian government members and Donald Trump's inner circle

Inquiry findings and whether it was still underway remained unclear, the Wall Street Journal reported on Sunday.

Flynn was among senior White House staff sworn in on Sunday.

He spoke to Russian Ambassador Sergey Kislyak over the phone at least twice before Trump was inaugurated, the White House has admitted, to set up a call between that country’s leader and the incoming president.

They also discussed a Russian-sponsored conference on the crisis in Syria and a plane crash that killed members of a Russian choir.

The first call, which the White House press secretary says took place on Dec. 28 but multiple sources, including another Trump aide, say happened a day later on, Dec. 29, was initiated by a text message sharing holiday greetings.

A follow-up call occurred a few days before Trump's inauguration, White House Press Secretary Sean Spicer said Monday. He told the Journal that Flynn told him no other calls took place during the transition.

Reuters had previously reported, based on three sources, that Flynn talked to Kislyak five times on Dec. 29 - the day Barack Obama's administration hit Russia with new sanctions in retaliation for a series of hacks it ordered in an effort to disrupt the U.S. presidential election.

A Trump aide confirmed to Reuters that the call took place on Dec. 29, too, though Spicer said it was a day before. WSJ also said it took place on the day the sanctions were levied.

Flynn, the Wall Street Journal said, was being probed by the FBI and intelligence agencies, as well as the Treasury Department, over his contact with the Russian ambassador and other figures.

Sarah Huckabee Sanders, a spokeswoman for the White House, told the Journal, 'We have absolutely no knowledge of any investigation or even a basis for such an investigation.'

Flynn raised eyebrows for receiving payment from the Russian TV network RT to join a gala celebration in Moscow in 2015, where he sat at a banquet table with President Vladimir Putin

It's not unusual for incoming administrations to have discussions with foreign governments before taking office.

But repeated contacts between Flynn and Russia just as Obama imposed sanctions would raise questions about whether Trump's team discussed - or even helped shape - Russia's response.

The national security adviser is not formally part of the cabinet but is usually one of the president's most influential counsels.

More broadly, Flynn's contact with the Russian ambassador suggests the Trump administration has already begun to lay the groundwork for its promised closer relationship with Moscow.

That effort appears to be moving ahead, even as many in Washington, including Republicans, have expressed outrage over intelligence officials' assessment that Putin launched a hacking operation aimed at meddling in the US election to benefit Trump.

As national security adviser, Flynn will work in the West Wing close to the Oval Office and will have frequent access to Trump.

Unlike Trump's nominees to lead the Pentagon, State Department and other national security agencies, Flynn's post does not require Senate confirmation.

Flynn, a veteran of America's wars in Iraq and Afghanistan, has courted controversy with extreme statements that critics say border on Islamophobia, but has taken a more flexible line on Russia and China.

He's come under scrutiny for his interactions with the Kremlin, however, since leaving his position as director of the Defense Intelligence Agency in 2014.