In an escalating dispute over how much allies should pay the U.S. to station troops on their soil, U.S. negotiators walked out on talks with South Korea in Seoul on Tuesday, as the two sides staked out vastly differing positions and accused the other side of being unreasonable.

The Trump administration reportedly is demanding that Seoul, a key ally, contribute around $5 billion for the coming year to cover the cost of the roughly 28,500 U.S. troops based in South Korea. That's more than five times what South Korea agreed to contribute earlier this year, and Seoul is pushing back hard.

The defense cost issue is putting stress not seen in years on a decades-old alliance and is drawing attention to the difficulty of putting a dollar value on the broad costs and benefits involved in military alliances.

At the heart of the issue is the Trump administration's contention that wealthy allies, including South Korea, are "freeloading" off the U.S. and spending too little for their own defense. Trump has suggested that U.S. troops could be withdrawn from the Korean Peninsula.

Seoul says the subject of removing forces has not come up in negotiations, and a law passed last year by the U.S. Congress prevents the president from reducing troop strength in South Korea without a determination by the secretary of defense that such a move is in America's national security interest.

Following Tuesday's aborted talks, State Department negotiator James DeHart told reporters that Seoul's proposals were "unresponsive" to U.S. demands. "We cut short our participation in the talks today in order to give the Korean side time to reconsider," he said.

DeHart's counterpart, South Korean negotiator Jeong Eun-bo, told reporters after the talks that the U.S had asked for a steep increase in South Korea's contribution, "while South Korea maintains the position that our burden should be within a mutually acceptable range."

U.S. demands have been met with a crescendo of criticism from across the Korean political spectrum.

"Seoul should wisely cope with Trump's move to rip off allies," advised an editorial in the largely conservative daily Dong-A Ilbo. The piece went on to grouse that the U.S., the superpower once seen as global cop, is retiring from that role "to launch a security company."

"We need to acknowledge that the U.S.-South Korea alliance is not in good shape," laments Park Hwee-rhak, a defense expert at Kookmin University in Seoul.

Park is a former South Korean army colonel, a self-described conservative and a supporter of the U.S.-South Korea alliance. He says he has long counseled his government to contribute more to shared defense costs, but even he finds the reported $5 billion demand unacceptable. He adds that the U.S. has failed to provide a detailed breakdown and rationale for its request.

The cost of any alliance can be broken down into its component parts: troops, equipment and operating expenses. South Korea currently covers about 40% of operating costs. It also provides land, construction and several thousand South Korean troops to augment U.S. forces.

The reported U.S. demand of $5 billion exceeds the $4.46 billion that the Defense Department puts as the cost of operations in South Korea in 2020.

Park argues that the U.S. and South Korea are not just two sides in a transaction. They're blood brothers, he says.

"We shed our blood in Vietnam and you shed your blood in South Korea and Vietnam together," he says. "So we supported your country and you supported us without calculating what kind of benefit I can get from this support."

South Korea sent troops to fight in Vietnam and has provided security in hot spots from Iraq and South Sudan to East Timor and Haiti.

The basing of U.S. forces abroad, meanwhile, gives Washington the ability to project power around the globe to secure commercial interests worth hundreds of billions of dollars.

"Roughly a third of U.S. trade comes out of northeast Asia," notes Bruce Bennett, a defense analyst at the Rand Corp., a U.S. government-funded think tank. "If there were a conflict in Korea, that trade would be severely disrupted."

Allies, Bennett notes, allow the U.S. to justify its projection of power for its own aims.

"They send troops to a variety of places in the world that provide stability and give us an ability to say we have a coalition doing these things," he says, "and it's not just U.S. imperialism where we're operating overseas."

He adds that increasing U.S. demands could prompt Seoul to cancel purchases of F-35 warplanes or other hardware, leading to the loss of American jobs.

Park argues that both the U.S. and South Korea have benefited from their alliance. But if the U.S. pulled out its troops, it would leave South Korea alone to face its neighbor to the north. For the U.S., this might be acceptable, but "for South Koreans," he says, it "would be a life-or-death situation."

While the Trump administration is pressuring Seoul to reach a deal this year, any agreement must still be ratified by South Korean lawmakers. Those lawmakers will be up for reelection next April. And while polls show that most South Koreans want U.S. troops to stay, a majority opposes a sharp increase in Seoul's contribution.

NPR's Seoul Bureau Assistant Se Eun Gong contributed to this story.