The Pentagon wants to keep its $US80 billion ($A89.68 billion) war chest even though most or all US troops are to withdraw from Afghanistan by the end of this year, officials say.
"Overseas contingency operations" funds are separate from the main Pentagon budget and have financed wars in Afghanistan and Iraq and counter-terrorism efforts elsewhere for more than a decade.
When President Barack Obama came into office, his deputies vowed to shift war spending back into the Pentagon's main budget.
But the Defense Department's proposed budget, released on Tuesday as part of the White House's annual spending plan, calls for $US79.4 billion in war funding for fiscal year 2015.
That represents only a small cut in the overseas operations fund, accorded $US85.2 billion this year.
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About 33,700 US troops remain in Afghanistan and the bulk of the force is due to depart by December.
The Obama administration hopes to keep a small contingent of up to 10,000 troops in the country beyond 2014 - if the Afghan government signs a bilateral security agreement.
In years past, administration officials predicted that war funding would decline significantly with the planned pullout of American forces from Afghanistan.
But instead, the Pentagon has used the category to finance maintenance and other items that would otherwise be slashed under automatic budget cuts.
Unlike the main base defence budget, the war funding is not subject to the mandatory spending limits enacted by Congress, known as sequestration.
Both the Pentagon and politicians last year moved about $US29 billion in funding out of the base budget to the overseas contingency fund (OCO), as a way of countering the effect of the automatic budget cuts.
The proposed main defence budget for 2015 is $US575 billion, less than the peak in military spending of $US691 billion in 2010 but still far beyond the 2001 budget of $US316 billion.
America's military spending far exceeds any other country, with China a distant second with an estimated $US132.2 billion and Russia at about $US68.9 billion, according to IHS Jane's consultant group.
But in its budget request, the Pentagon said it faced a shortage of funding for modernising aircraft, training and maintaining bases.
Pentagon chief Chuck Hagel already laid out the main priorities of the defence budget last week, proposing a cut in the size of the US Army while keeping up investments in hi-tech weaponry.
The budget calls for scaling back the army from 520,000 troops to about 440,000 to 450,000 soldiers by 2017, and retiring older aircraft include the A-10 "Warthog" and the U-2 spy plane.
The budget would fund the new F-35 fighter jet, more Global Hawk surveillance drones, armed Reaper drones, warships, submarines and the first new aerial KC-46A refuelling tankers.
The Defense Department also released a major strategic review, conducted every four years, that endorsed the budget plan and called for preserving the military's ability to move forces rapidly around the world despite an era of "fiscal austerity".
