Washington, Sep 19 (IANS): US investigators wiretapped President Donald Trump's former campaign chairman Paul Manafort under secret court orders before and after the election, the media reported.

"The conversations between Manafort and Trump continued after the President took office, long after the FBI investigation into Manafort was publicly known," informed sources told CNN on Monday night.

"They went on until lawyers for the President and Manafort insisted that they stop.

"It's unclear whether Trump himself was picked up on the surveillance," the sources added.

Some of the intelligence collected includes communications that sparked concerns among investigators that Manafort had encouraged the Russians to help with the campaign.

The sources, however, cautioned that the evidence was not conclusive.

Special counsel Robert Mueller's team, which is leading the investigation into Russia's involvement in the election, has been provided details of these communications.

A secret order authorised by the court that handles the Foreign Intelligence Surveillance Act (FISA) began after Manafort became the subject of a Federal Bureau of Investigation (FBI) probe that began in 2014.

It centred on work done by a group of Washington consulting firms for Ukraine's former ruling party, the sources told CNN.

"The probe was discontinued at some point last year for lack of evidence but the FBI restarted it after obtaining a new FISA warrant that extended at least into early this year."

Manafort previously has denied that he ever "knowingly" communicated with Russian intelligence operatives during the election and also has denied participating in any Russian efforts to "undermine the interests of the United States".