By David Morgan

WASHINGTON, Feb 25 (Reuters) - Top Pentagon officials have had to agree in writing to keep this year's budget talks a secret as they confront thorny questions about the future of expensive U.S. weapons programs, a spokesman said on Wednesday.

Defense Secretary Robert Gates took the unusual step of requiring nondisclosure agreements of all senior officials who wanted to participate in the fiscal debate, including the Joint Chiefs of Staff.

President Barack Obama, who inherited a recession and a trillion-dollar deficit when he took office on Jan. 20, will send his first budget proposal to Congress on Thursday but a comprehensive fiscal plan is not expected until April.

Some budget watchers believe the Pentagon's base budget could rise as much as 4 percent from a current $515 billion but would remain well below the $581 billion initially sought for the new fiscal year by the Bush administration.

Up to now, the wars in Iraq and Afghanistan have been funded by separate supplemental bills and not through the base budget.

Total U.S. government spending stands at more than $3 trillion a year.

"Everybody who's participating in this process -- these are the highest-ranking people in this department -- were asked to sign," Pentagon press secretary Geoff Morrell told reporters.

Obama has pledged to break with the unprecedented secrecy of the Bush administration and allow greater openness in government activities.