US Secretary of State Mike Pompeo and Defense Secretary Mark Esper have made an emphatic call for South Korea to pay more for the upkeep of 28,500 American troops stationed in the country.

They made the appeal in a rare joint commentary in the Wall Street Journal on Thursday (US time), after the two sides ended their sixth round of talks in Washington to negotiate the Special Measures Agreement, the cost-sharing deal for the stationing of the US Forces Korea.

t the SMA talks on Tuesday and Wednesday, the negotiators failed to reconcile differences, with Seoul striving to maintain the decades-old SMA framework and Washington demanding a change in the accord to expand its coverage.

"These narrowly defined costs are only one part of the picture.

America's contributions to South Korea's defense in this highly technological age -- including some advanced capabilities Seoul still needs to acquire -- far exceed the cost of US 'boots on the ground' and constitute a far larger burden for the American taxpayer than meets the eye," they wrote in the commentary.

"The current special measures agreement captures only a portion of the cost of defending South Korea. The US believes it should cover more. As we improve the burden-sharing arrangement, both sides will benefit," they added.

The scope of the SMA was a major fault line in the negotiations as Seoul has remained firm that the basic framework of the agreement should remain intact to ensure the purpose of its payments -- sharing the cost of stationing the USFK.

Currently, the SMA has three categories for Korea's financial contributions: the partial costs for South Korean employees in USFK installations, the construction of some military facilities and logistical support.

But Washington has demanded the SMA should cover a full range of costs associated with American troops' role for the defense of South Korea, including those related to their rotational deployments to the peninsula.

The two secretaries stressed that the vast majority of South Korea's payments for the USFK will benefit the local economy -- a key rationale to ramp up pressure on Seoul to shoulder a greater share.

They also made it clear that America's presence on the peninsula has "enabled South Korea to develop a vibrant democracy and the world's 12th largest economy."

"More than 90% of South Korea's cost-sharing contributions currently go right back into the local economy in the form of salaries for South Korean nationals employed by US Forces Korea, construction contracts, and other services purchased locally to sustain an American presence. It's good for both nations," they said.

Noting the "tough" SMA negotiations, the secretaries also said that South Korea's taking on a greater share of the load will ensure the alliance remains the "linchpin of peace and prosperity" on the Korean Peninsula, in Northeast Asia and across the world.

"The US remains firmly committed to reaching a mutually beneficial and equitable agreement that will strengthen the alliance and combined defense far into the future," they said. (Yonhap)