President-elect Donald Trump's national security adviser, Michael Flynn, and Russia's ambassador to the U.S. have been in frequent contact in recent weeks, including on the day the Obama administration hit Moscow with sanctions in retaliation for election-related hacking, a senior U.S. official said on Friday.

After initially denying that Mr Flynn and Ambassador Sergey Kislyak spoke Dec. 29, a Trump official said the transition team was aware of one call on the day President Barack Obama imposed sanctions.

It's not unusual for incoming administrations to have discussions with foreign governments before taking office. But repeated contacts just as Obama imposed sanctions would raise questions about whether Trump's team discussed - or even helped shape - Russia's response.

Russian President Vladimir Putin unexpectedly did not retaliate against the U.S. for the move, a decision Trump quickly praised.

More broadly, Mr Flynn's contact with the Russian ambassador suggests the incoming 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 U.S. election to benefit Mr Trump.