Eric Holder has been accused of misleading Congress over his involvement in aggressively investigating journalists for security leaks, forcing the White House to repeatedly defend its embattled attorney general.

Republican critics seized on fresh information about attempts to obtain classified emails sent to a Fox News journalist in 2010 and said Holder should have mentioned it when asked about pursuit of such leaks before the House judiciary committee this month.

Holder denied any knowledge of attempts to prosecute journalists using the Espionage Act in such cases, suggesting it would be a step too far.

"With regard to potential prosecution of the press for the disclosure of material, that is not something that I have ever been involved in, heard of, or would think would be a wise policy," he said during the 15 May hearing.

But new reports into the government pursuit of private emails belonging to James Rosen, chief Washington correspondent for Fox News, suggest it was approved at the highest levels in the Justice Department following discussions with Holder, who may have personally signed it.

Crucially, an affidavit in support of the search warrant said there was "probable cause to believe that the reporter has committed violation [of the Espionage Act] at the very least, either as an aider, abettor and/or co-conspirator" in obtaining US assessments of North Korea's nuclear programme.

Congressional opponents wrote to Holder on Wednesday suggesting this implied threat to pursue criminal charges against Rosen clashed with his statement to them.

"The media reports and statements issued by the [Justice] Department regarding the search warrants for Mr Rosen's emails appear to be at odds with your sworn testimony before the committee," wrote committee chair Bob Goodlatte, who demanded Holder clarify his role.

The White House issued a robust response to the letter, but is under growing pressure over the affair, which has damaged relations with the media following a similar government investigation into phones used by Associated Press reporters.

"The president believes that the attorney general is doing a good job and has confidence in the attorney general," said spokesman Jay Carney.

He drew a distinction between Holder's reference to his lack of knowledge of "potential prosecutions" of reporters and the Rosen case where no charges were brought.

"[Holder] was talking about prosecutions. There is an extremely large distinction between what's at issue here and prosecution," said Carney. "Self-evidently the charge [that Holder misled Congress] is inaccurate. I see no conflict between what he said and the published reports."

Nevertheless, it seems likely that the Justice Department will be forced to respond to the House committee to clarify Holder's role. The attorney general is also due to meet media representatives later this week to discuss the broader issues around balance press freedom with national security.

"It is important not to lose sight of the fact that we are talking about classified information, the leaking of which is a crime," added Carney. "Unless there is a suggestion that we should just make all information available to everyone, including al-Qaida, there has to be some process in place that allows for the protection of that classified information."