The FBI investigated Trump campaign advisor Carter Page last year over worries he could have been working for Russia, the Washington Post reported on Tuesday.

In the first confirmation that the government conducted surveillance on Donald Trump's team, the Federal Bureau of Investigation obtained a rare warrant to monitor Mr Page's communications from the top secret Foreign Intelligence Surveillance Court, the Post said.

The so-called FISA warrant was issued after the FBI told the court that there was probable cause to believe the former Moscow-based banker was working for Russia.

FISA warrants are almost never made public, and the Post cited unnamed law enforcement and other officials in its report. The FBI did not respond to requests to comment on the report.