The FBI obtained a secret court order last summer to monitor the communications of Carter Page, an adviser to Donald Trump, because the government suspected he was acting as a Russian agent, a report claims.
The Washington Post, citing unnamed law enforcement and other US officials, said the surveillance application laid out the basis for believing Mr Page had knowingly engaged in intelligence activities on Moscow's behalf.
The newspaper said the application included contacts Mr Page had with a Russian intelligence operative in 2013.
He has denied having improper ties to Russia. He said he is "happy" the court order has been revealed and blames the Obama administration for trying to "suppress dissidents who did not fully support their failed foreign policy".