WASHINGTON (Reuters) - Defense Secretary Leon Panetta ordered senior Pentagon officials on Thursday to begin monitoring major U.S. news media for disclosures of classified information in an effort to stop the release of government secrets after a series of high-profile leaks.

The announcement came hours after Panetta and other senior defense officials appeared before a closed-door hearing of the House of Representatives Armed Services Committee to discuss recent disclosures of classified security information.

Reports about U.S. cyber warfare against Iran, procedures for targeting militants with drones and a double agent who penetrated a militant group in Yemen have angered U.S. lawmakers. Some have charged the leaks were timed to benefit President Barack Obama's re-election bid.

Representative Buck McKeon, the Republican chairman of the House panel, told a news conference later that he did not believe the Pentagon was behind the leaks and that Panetta and the other officials were taking the issue seriously.

"Both the chairman and I were very convinced that Secretary Panetta and all of the folks at the Pentagon are taking it seriously, are trying to mitigate the damages and prevent it in the future," said Adam Smith, the top Democrat on the panel.

Panetta, Army General Martin Dempsey, the top uniformed military officer, and chief Pentagon lawyer Jeh Johnson appeared before the committee to answer questions. McKeon said they agreed the recent leaks had caused damage, but did not elaborate.

In addition to the media monitoring ordered by Panetta, the Pentagon said it had taken a number of other steps in recent months to improve information security.

The measures included improved training for handling classified information, the publication of a manual with clear instructions on what constitutes an unauthorized disclosure and the creation of an online security incident reporting system.

"The department is continuously improving its security posture and overall capability to prevent unauthorized disclosures," the Pentagon said in a statement disclosing the recent security changes.

McKeon said the House panel was "concerned about the leaks that have come out over the years and accelerated, it seems, over the last few months."

Although he said he did not believe the Pentagon was the source of the most recent leaks, McKeon declined to comment on speculation the White House was responsible.

Senator John McCain, Obama's Republican opponent in the 2008 presidential election, has suggested some of the leaks may have been calculated to boost the Democratic president's re-election efforts - a charge the White House emphatically denies.

Pentagon Press Secretary George Little told a briefing on Thursday that Panetta, Dempsey and members of the House committee were of one mind about the leaks.

"The unauthorized disclosure of classified information is truly disturbing," he said. "It's of concern to the secretary, and I think members on the Hill express similar concern. And the secretary is clearly prepared to try to address the problem inside the department."

McKeon said the U.S. government was "actually doing pretty good" in handling its secrets, given the fact that 4 million people had some form of classified clearance.