Defense Secretary Jim Mattis has fired off letters to lawmakers listing his objections to the two versions of the 2018 National Defense Authorization Act, including his fear that language in the bill limits the ability of the U.S. to take decisive action against cyber threats.

Mattis sent letters to the chairmen and ranking members of the House and Senate armed services committees over the NDAA, which is now being hammered out by House and Senate negotiators.

The letter, sent Tuesday, is known at the Pentagon as the "heartburn letter," because it lays out the issues that are causing heartburn to the secretary of defense.

A copy has been provided to the Washington Examiner.

In the letter, Mattis cited what he called a "conventional approach applied to an unconventional problem," in objecting to language in the Senate version of the bill that he said would hamstring the U.S. in taking decisive action against cyber threats requiring the U.S. to give prior notification to foreign governments before acting.

Mattis also listed as his "primary concern" the budget caps imposed by the 2011 Budget Control Act.

"No enemy in the field has done more to harm the warfighting readiness of our military than sequestration," Mattis wrote, repeating a line he has often used in congressional testimony. "Current caps continue to unnecessarily defer critical maintenance, limit aviation availability, delay modernization, and strain our men and women in uniform," he said.

And he made another plea for the authority to close unneeded bases and get rid of excess infrastructure with another round of base realignment and closings, or BRAC.

"We have studied shortcomings of previous rounds and are confident the savings generated by a new BRAC in 2021 would save $2 billion or more annually," money Mattis said is "urgently needed" to fund readiness and modernization.

Mattis also opposes the creation of a space corps as a new military service, changes to the military health system, and a cap on the cost of military construction projects.

Mattis said he would like to see language to support higher "reasonable" pharmacy copays, which he said could bring in $6 billion over the next several years, and new rules to make it easier to bring in workers for military construction projects in Guam and the Northern Mariana Islands.

"Left unaddressed, these items impact my priorities of restoring military readiness while building a more lethal force, strengthening alliances, and bringing business reform to the Department of Defense," Mattis wrote.

Congressional negotiators have until December to hash out differences between the Senate's $700 billion NDAA and the House's $696 billion version.

Until then, the Pentagon is operating under a continuing resolution that limits spending to last year's level.