Michael Flynn, President-elect Donald Trump's choice for national security adviser, held five phone calls with Russia's ambassador to Washington on the day the United States retaliated for Moscow's interference in the U.S. presidential election, three sources familiar with the matter said.

The calls occurred between the time the Russian Embassy was told about U.S. sanctions and the announcement by Russian President Vladimir Putin that he had decided against reprisals, said the sources. They spoke on condition of anonymity, citing internal U.S. government deliberations about the issue.

The calls raised fresh questions among some U.S. officials about contacts between Trump's advisers and Russian officials at a time when U.S. intelligence agencies contend that Moscow waged a multifaceted campaign of hacking and other actions to boost Republican Trump's election chances against Democrat Hillary Clinton.