President-elect Donald Trump's national security adviser took a call from Russia's ambassador last month on the same day President Barack Obama announced new sanctions against Russia for allegedly interfering in the U.S. election.

The Washington Post, Reuters, and the Associated Press reported and a Trump spokesman confirmed on January 13 that the adviser, Michael Flynn, had at least one call with Ambassador Sergey Kislyak on the same day the sanctions and the expulsion of 35 Russian diplomats were announced.

The new sanctions and expulsions were in response to U.S. intelligence findings that Russian-backed hackers had intruded on U.S. political parties and hacked e-mails as well as complaints from U.S. officials about a multiyear campaign of harassment of its diplomats in Russia.

The Trump campaign originally said the calls occurred the day before the sanctions were announced on December 29, but later conceded that one call occurred on the day of the announcement. Reuters cited three officials in saying five phone calls occurred that day.

The January 13 news reports about Flynn's contact with Kislyak, first reported by the Washington Post a day earlier, added to the growing concern in some Washington circles about the nature of the interactions between members of Trump's campaign team and Russian officials.

The Post cited an unidentified U.S. government official as saying that the calls occurred on December 29. It quoted an unidentified Trump official as saying Flynn had earlier calls with Kislyak to express condolences for the shooting of Russia's top diplomat in Turkey and the crash of a Russian plane carrying a famed choir.

During the call, the two men discussed setting up a call between Trump and Putin after Trump's inauguration on January 20, Trump spokesman Sean Spicer said.

'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 on how to initiate and schedule that call. That was it, plain and simple,' Spicer told reporters.

But AP and Reuters both cited unidentified U.S. officials as saying that Flynn and Kislyak spoke on December 29, when Obama announced the new sanctions and the expulsion.

Many observers had expected Moscow to retaliate to the sanctions and the expulsions and kick a similar number of U.S. diplomats out of Russia -- something that happened routinely during the Cold War.

Putin, however, announced that he would not respond tit-for-tat, a move that Trump praised on Twitter, calling the Russian president 'very smart.'

Suspicions about ties to Russia have dogged the Trump campaign throughout the election season. The campaign's manager, Paul Manafort, left after news reports on his alleged financial dealings with the party of Viktor Yanukovych, the former Ukrainian president who was backed by the Kremlin.

After the November 8 election, Deputy Foreign Minister Sergei Ryabkov was quoted by the Interfax news agency as saying the Russian government had been in touch with Trump's political team during the campaign.

Flynn has faced criticism for his participation last year in a gala celebration in Moscow for the government-funded TV network now known as RT. Photographs from the event showed him seated at a banquet table directly next to Putin.

Asked about the reports, White House spokesman Josh Earnest said he didn't know if administration officials were informed of Flynn's call in advance.

He also declined to say whether the call between Flynn and the Russian ambassador would be objectionable.

'It depends on what they discussed,' Earnest told reporters on January 13.

A 218-year-old law known as the Logan Act prohibits private citizens from negotiating with foreign governments having a dispute with the United States without government authorization.

With reporting by the Washington Post, Reuters, and AP