Washington ? Even as it touts U.S. efforts to sharply reduce its number of nuclear warheads, the Obama administration plans to increase spending on the aging nuclear weapons infrastructure to levels reminiscent of the Cold War, a new budget document shows.

A 20-year spending plan from the agency that manages the nuclear arsenal shows that the administration wants to hike nuclear weapons spending to an average of about $8 billion a year in coming years, compared with recent spending levels of $6 billion to $7 billion a year.

The National Nuclear Security Administration plan is drawing criticism from some arms control groups, who contend the increased spending is unjustified and may prompt other world powers to doubt President Barack Obama’s pledge to reduce American reliance on nuclear weapons.

“We have to think carefully about what signal we’re sending to other countries,” said Hans Kristensen of the Federation of American Scientists.

Advocates of the plans within the administration and Congress contend the spending increase is overdue to modernize the nuclear support complex, parts of which haven’t been updated since World War II.

Administration officials also argue that even as they reduce the number of U.S. nuclear warheads, they need to bolster the government’s ability to increase weapons production quickly if a new threat arises.

Obama has made reduction of the nuclear arsenal a central element of his foreign policy agenda. The president hopes that by shrinking the U.S. arsenal, the administration can convince nuclear states to cut back their stockpiles, and encourage non-nuclear states to decide against starting nuclear weapons programs.

The Senate is now considering the New Start nuclear treaty, a pact with Russia that would reduce the number of active long-range nuclear warheads by about 30 percent.

The increase in nuclear spending has political value for the White House: Administration officials hope it will help win the votes of conservative senators who have reservations about the treaty with Russia.

Kristensen said the plan suggests that the agency is preparing to draw down the U.S. nuclear stockpile to between 3,000 and 3,500 weapons by 2021, a drop of 30 to 40 percent from the current stockpile of about 5,000 weapons. A spokeswoman from the agency declined to comment on the estimate.

Other analysts said such a drop would be consistent with the kind of reduction that Obama has already described. “It’s not surprising,” said Daryl Kimball of the Arms Control Association.

Reductions already under way will reduce the stockpile to 4,700 weapons by 2012. In the past four-plus decades, the U.S. arsenal already has fallen 84 percent from its peak of 31,000 warheads in 1967.

But in the world of nuclear weapons, there’s a lot more to pay for than just warheads. The plan calls for the United States to spend $175 billion from 2010 to 2030 on new weapons production, testing and simulation facilities, and on extending the life of nuclear weapons in the arsenal.

And that’s not all: The Pentagon’s spending to maintain and operate the equipment that delivers the warheads — missiles, bombers and submarines — is not included.

Spending for the weapons complex would peak between 2014 and 2018 under the plan.

Henry Sokolski, a Pentagon official during the George H.W. Bush administration, cautioned that despite the administration’s planning, the nuclear stockpile ultimately could be vastly different from what is being forecast today.

“For the first year or two any spending plan for the executive branch will be quite firm and useful,” said Sokolski, now with the Nonproliferation Policy Education Center in Washington. But because of changing threats and fiscal realities, in future years “they become exponentially more speculative, no matter what they say.”