The Pentagon said Wednesday that top military officers and civilians had to sign a letter promising to keep details secret as they work on the military's budget.

Defense Department press secretary Geoff Morrell told reporters that Secretary Robert Gates made the unusual request out of concern for national security. He said the department didn't want any leaks to "unravel" the budget process. 

"This is highly sensitive stuff involving programs costing tens of billions of dollars, employing hundreds of thousands of people and go to the heart of national security," he said. "And so he wants this process to be as disciplined and as forthright as possible. 

"And he thinks that by having people pledge not to speak out of school, if you will, on these matters while they are a work in progress, that you'll create a climate in which you can ultimately produce a better product, because people can speak candidly with the confidence that it will not be leaked," he said. 

Gates remained as secretary under Obama after serving under President Bush, but this year is the first time he is requiring the non-disclosure statements.

The entire Joint Chiefs of Staff signed on, promising not to leak information while the budget is being put together. Morrell said Gates also signed it. 

The agreement, a copy of which was obtained by FOX News, commits those signing it not to divulge "budget-related information" including planning, programming and other aspects. 

The agreement states that a "significant factor in the successful and proper preparation and completion of the president's budget is the strict confidentiality that must be observed," and that a failure to comply with that confidentiality could "compromise" the administration's ability to draft and submit its spending plan. 

Morrell also disclosed Wednesday that the Pentagon is expecting to have a war supplemental appropriation for fiscal 2010, which would cover additional funding for the wars in Iraq and Afghanistan -- even though Obama made a reference in his speech Tuesday to Congress to scaling back the practice of supplemental appropriations.

Obama said during the speech that the government is suffering from a "deficit of trust," and, "I am committed to restoring a sense of honesty and accountability to our budget. ... For seven years, we have been a nation at war. No longer will we hide its price."

The hidden cost line was a direct reference to the Defense Department's war supplemental that is approved each year to fund the wars in Iraq and Afghanistan long after the base budget is set. In fiscal 2008, $187 billion was added as the supplemental. So far in fiscal 2009, $66 billion has been added for the wars and another supplemental of greater value is expected before the year's end.

War spending is dictated by the situation on the ground and remains a moving target, a defense official told FOX News.

The Pentagon will try to include all "predictable" war costs in the base budget and wants to eventually move away from supplemental budgets, Morrell said, but in fiscal 2010 there will be costs "above and beyond" early estimates. 

"It's extremely difficult for us to predict what our level of commitment is going to be in either theater, let alone theaters that we haven't potentially thought of, God forbid, a year, two years, let alone 10 years from now," he said. "So we are trying to be as helpful as possible in this process, but some of this stuff is not known at this point."

White House Press Secretary Robert Gibbs said the president's budget will be "honest and transparent." 

"It might be hard to throw a dart and hit the exact number on the board that accompanies the exact total of spending in that fiscal year," Gibbs said. "But there wasn't even a picking up of the dart and throwing it at the board (in the last administration)."

FOX News' Jennifer Griffin and Justin Fishel and The Associated Press contributed to this report.