"Overall, sequestration will put us on a path toward a hollow force and inflict serious damage on our national security," Panetta wrote.
Panetta was flying Wednesday to Brussels to attend a NATO defense ministers meeting. Spokesman George Little told reporters en route that Panetta would tell his counterparts that across-the-board budget cuts will hurt not only the U.S. military but also the ability of NATO to respond to crises.
Little said the Pentagon is also discussing the possibility of not being able to send military units on planned rotations to various places around the world. In anticipation of cuts, the Pentagon has already decided not to send one aircraft carrier back to the Persian Gulf, reducing the U.S. presence there to one carrier.
The Pentagon has begun discussing details of the furloughs with defense worker union officials.
President Barack Obama has exempted military personnel from furloughs.
Obama was continuing to pressure Republican lawmakers to avert the automatic cuts by supporting a Senate Democratic plan that would replace the immediate cuts with a mix of spending reductions and tax increases. He was conducting interviews with local television in eight markets: Boston, Charleston, S.C., Baltimore, Oklahoma City, Okla., Wichita, Kan., San Antonio, Texas, San Francisco, and Honolulu.
"I don't know why it is in this town folks leave stuff to the last minute. You know, there's no other profession, no other industry, where people wait until the 11th hour to solve these big problems," Obama told WJZ, the CBS affiliate in Baltimore.
The only civilian Pentagon workers who would be exempt from furloughs would be Senate-confirmed political appointees such as the defense secretary and deputy defense secretary, as well as a relatively small number of workers deemed essential to protect the safety of defense property and personnel.
Panetta said the administration is still working with Congress to avoid automatic budget cuts by reaching agreement on a deficit reduction plan.
Associated Press writers Donna Cassata, Lolita C. Baldor and Bradley Klapper contributed to this report.