US commanders told lawmakers on Thursday that deep cuts to military spending will leave American forces ill-prepared for combat and risk higher casualties on the battlefield.

The chiefs of all the armed services issued their most serious warning yet on the effect of automatic budget cuts, saying the funding reductions will result in a smaller force that will begin to lose its technological edge.

But even under the automatic cuts, the US military's current budget of roughly $496 billion far surpasses any other country's defense spending, representing about 40 percent of the world's military expenditures.

The top brass have issued previous appeals on the automatic cuts but there is little sign Congress is ready to lift the sweeping budget reductions, known as sequestration, which were triggered by political deadlock.

"We will have fewer forces, arriving less trained, arriving later to the fight," said General James Amos, commandant of the US Marine Corps.

The lack of training and slower deployment would give adversaries more time to prepare and probably prolong any conflict, he said.

"This is a formula for more American casualties," Amos told the Senate Armed Services Committee.

The bleak picture painted by the generals came a day after Pentagon chief Chuck Hagel warned the military would have to be smaller and sacrifice a degree of readiness to adapt to the budget squeeze.

In a speech Wednesday, Hagel called the automatic cuts "too fast, too much, too abrupt, and too irresponsible."

The military chiefs have repeatedly appealed to lawmakers to lift the automatic reductions and Thursday's testimony marked the first time the officers raised the prospect of higher casualties.

Amos said the Marine Corps would have to shrink from a force of 202,000 to 174,000, which would mean that in a war, troops in the field would not be relieved and would stay on the ground until the conflict ended.

Amos invoked the Korean War, when American forces were seen as ill-prepared for a major conflict, saying the United States was facing a similar "hazard" as it slashes military spending.

'Lower readiness levels'

The US Army's chief of staff, General Ray Odierno, also warned that the dramatic budget cuts would mean putting troops into combat without enough training, raising the danger of more soldiers dying on the battlefield.

"In the event of a crisis, we will deploy these units at significantly lower readiness levels," the four-star general said.

"Our soldiers are adaptive and agile. Over time they may accomplish their mission, but their success will come with the greater cost of higher casualties," he said.

The budget reductions would make it "very difficult" to carry out even one "sustained major combat operation," Odierno said.

Due to the sequester cuts, the army will have to scale back its force to 420,000, after a war-time high of 570,000 troops, he said.

The army had previously planned to shrink to 490,000 in the next few years.

The downsizing represented an 18 percent overall reduction in US Army forces, including a 26 percent cut to active duty troops.

The general said that unlike previous drawdowns, the country was cutting back its ground force while a war was still in progress, citing the more than 50,000 troops now in Afghanistan.

Under automatic reductions mandated by Congress, the Pentagon absorbed $37 billion worth of cuts in the previous fiscal year and faces the prospect of another $52 billion in additional cuts.

The air force has already said it expects to cut back personnel by about 25,000 airmen and the navy's chief told lawmakers that the country's fleet of warships would be dramatically reduced over the next decade if the automatic budget reductions continue.

The budgets for each armed service traditionally have been roughly equal. But Hagel told the New York Times in an interview published Wednesday that budget pressures meant he would not insist on each branch of the military getting an equal share.

The automatic cuts create difficulties because they require the Pentagon to cut all budget accounts -- except personnel -- by an equal amount, said Lawrence Korb, a former Pentagon official and a senior fellow at the Center for American Progress.

The total dollar figure for the Pentagon's budget under sequestration is equivalent to spending levels in 2007 and "in real terms is higher than it was on average during the Cold War," he said.