WASHINGTON, Sept 18 (Reuters) - U.S. investigators wiretapped former Trump campaign chairman Paul Manafort under secret court orders before and after the 2016 election, CNN reported on Monday.

The New York Times, citing two people close to the case, also reported that prosecutors told Manafort they planned to indict him. Federal agents had raided Manafort's Virginia house in July.

Manafort is one of several close advisers who helped President Donald Trump win the 2016 election and who are now being investigated as part of Special Counsel Robert Mueller's probe of alleged Russian interference in the U.S. vote.

Manafort became Trump's campaign manager in June 2016 but was forced to resign two months later amid reports of his business relationship with the Kremlin-backed former Ukrainian leader, Viktor Yanukovich.

Manafort's attorney and the White House did not immediately respond to Reuters' requests for comment on the stories.

The secret court that oversees warrant requests under the Foreign Intelligence Surveillance Act initially authorized monitoring of Manafort but discontinued it at some point in 2016 for lack of evidence, CNN said, citing one of its sources.

The FBI resumed surveillance under a new FISA warrant at some point last year and continued to monitor him into 2017, CNN said. The second warrant was obtained as part of the FBI's investigation into ties between Trump campaign associates and Russian operatives, it said.

FISA warrants require the approval of top FBI and Justice Department officials, and law enforcement has to demonstrate to the court that there is reason to believe that the subject of the investigation may be acting as an agent of a foreign power.

CNN said interest in Manafort deepened because of intercepted communications between him and Russian operatives, and among the Russians. The government eavesdropping continued into 2017, including a period when Manafort was known to talk to Trump after he became president. CNN said it was unclear if Trump was picked up on the surveillance.

The FBI was not listening in June 2016 when Donald Trump Jr., Manafort and top White House adviser Jared Kushner met with a Russian lawyer who had promised to deliver negative information on Trump's rival, Democrat Hillary Clinton, CNN said.

Russia has denied interfering in the U.S. election and Trump has denied there was collusion.

(Reporting by David Alexander; Editing by Peter Cooney)