Democratic Sen. Al Franken said he's concerned about a report that incoming National Security Adviser Michael Flynn spoke several times with Russia's ambassador to the United States on the day President Obama announced new sanctions.

"I'm concerned with all of this, of course," Franken said on MSNBC Friday. "This was a very big part of what happened in this election, this is unprecedented, this kind of foreign interference and the Trump team's relationship [with Russia] is of import."

On Thursday evening, the Washington Post cited an unnamed government official who said Flynn spoke with Russian Ambassador Sergey Kislyak "several times" on Dec 29. That's the same day Obama announced sanctions against Russia and expelled multiple Russian officials from the country in retaliation for Russian hacking into emails belonging to national Democrats.

Incoming White House Press Secretary Sean Spicer said the extent of Flynn's contact with Kislyak was wishing the ambassador a Merry Christmas and then later on coordinating a call between Trump and Russian President Vladimir Putin after the inauguration.

"The call centered around the logistics of setting up a call with the president of Russia and the president-elect after he was sworn-in, and they exchanged logistical information and how to do that," Spicer said.

Flynn developed a close relationship with Russia since retiring from the Army as a lieutenant general, visiting the country on multiple occasions and appearing on the state-run Russia Today channel. He spoke at an event in Russia honoring the channel, described as a purveyor of "international propaganda" by American intelligence, and sat next to Russian President Vladimir Putin at the event.

The Minnesota Democrat said the 35-page unverified report released this week alleging connections between the Trump campaign and Russian intelligence needs to be investigated.

"There has to be an investigation into these charges that his campaign coordinated with Russia," Franken said. "He should be more transparent. He has to release his taxes."

Franken said Trump and his team's trust of Russia and the willingness to believe Russian officials over U.S. intelligence officials is worrisome. There has to be something causing Trump to praise Putin or the Russian government and give more weight to their statements than American officials, he said.

He speculated there may be a financial reason behind Trump's overtures to the Kremlin.

"If you're a developer that has gone bankrupt several times and left your creditors holding the bag, you have to get money from somewhere," he said.