WASHINGTON — A senior U.S. official says the Obama administration is aware of frequent contacts between President-elect Donald Trump's top national security adviser and Russia's ambassador to the United States.

The official says the contacts happened on the day President Barack Obama expelled dozens of Russian officials from the U.S. and imposed sanctions as punishment for election-related hacking. The official says the administration is also aware of contacts between Trump adviser Michael Flynn and Ambassador Sergey Kislyak at other times as well.

The day after Obama announced sanctions, Russian President Vladimir Putin said he would not retaliate. Trump praised him for his restraint.

Flynn's conversations were first reported by Washington Post columnist David Ignatius. The official was not authorized to confirm the contacts publicly and spoke on condition of anonymity.