WASHINGTON — The House Armed Services Committee chairman, speaking after a secret briefing from top defense officials on Thursday, said he did not believe the Pentagon was responsible for recent national security leaks.

‘‘I feel pretty secure they were not’’ from the Pentagon, Representative Howard ‘‘Buck’’ McKeon, Republican of California, told reporters after the three-hour closed hearing with Defense Secretary Leon Panetta, Joint Chiefs of Staff chairman General Martin Dempsey and top Pentagon counsel Jeh Johnson.

McKeon would not say where he believed the leaks came from, but some Republicans have speculated that the White House was responsible for the leaks to improve President Obama’s chances of reelection.

They have demanded a special counsel, removing an investigation of the leaks from two US attorneys selected by Attorney General Eric Holder.

The chairman said the officials ‘‘assured us they are proceeding to try to limit those leaks,’’ adding that the leaks have ‘‘the potential for causing serious harm.’’ He said all three officials agreed the leaks have damaged national security.

The top Democrat on the committee, Representative Adam Smith of Washington state, said he also was convinced the Pentagon was taking the issue seriously.

A separate Defense Department document obtained by The Associated Press spells out the steps the Pentagon has taken to reduce the unauthorized disclosure of classified information. Among the 10 steps were better training of personnel if they suspect a threat from an insider or observe a leak and an online reporting system for significant security incidents.