The Committee to Protect Journalists says it is concerned about reports of the seizure of emails and phone records from New York Times reporter Ali Watkins, calling it the first known such incident in the Trump Administration.
The President has branded the Times one of the "enemies of the people" in league with his Democratic opponents to undermine his Administration.
The Times reported that Watkins' data was obtained from telecom companies as part of a leak investigation.
The Obama Administration set a record for leak-related prosecutions, CPJ pointed out, but it is afraid it could be an "an opening salvo" for the Trump Administration "in an ongoing battle over reporters' ability to protect their sources."
"In order to perform their public accountability function, journalists must be able to protect their confidential sources. Efforts by government that undermine this ability therefore represent a fundamental threat to press freedom," said CPJ's North America Program coordinator Alexandra Ellerbeck in a statement.
Former Senate Intelligence Committee aide, James A. Wolfe, has been charged with lying to government investigators about contacts with three reporters, the Times said, after authorities said he made false statements to the FBI about providing two reporters sensitive committee information.
"The seizure suggested that prosecutors under the Trump administration will continue the aggressive tactics employed under President Barack Obama," and CPJ feared that would be the case.
The President weighed in with with reporters Friday, according to a transcript supplied by the White House:
Q: "Are you glad they caught a leaker?" President: "It's very interesting that they caught a leaker in a very important -- it's a very important leaker. So it's very interesting. I'm getting information on it now. Happened last night. It could be a terrific thing.
"I believe strongly in freedom of the press. I’m a big, big believer in freedom of the press, but I'm also a believer in classified information. Has to remain classified. And that includes Comey and his band of thieves who leak classified information all over the place.
"So I’m a very big believer in freedom of the press, but I’m also a believer that you cannot leak classified information."