The commander of US troops in South Korea today called for negotiators to quickly resolve a cost-sharing dispute that has put more than 4,000 local workers on unpaid leave, with South Korean officials suggesting an agreement could come soon.

About half of the nearly 9,000 South Koreans employed by the US military command were placed on unpaid leave starting today after the two countries failed to agree on a new pact months after the last agreement expired at the end of last year.

The dispute follows a US demand for Seoul to pay up to US$5 billion a year to support the troop presence, up from US$870 million under last year's agreement.

"This is an unfortunate day for us, it's unthinkable, it's heartbreaking," General Robert Abrams, commander of United States Forces Korea (USFK), said in a video statement.

"The partial furlough of (Korean national) employees is not what we envisioned or hoped would happen."

On Tuesday, South Korean negotiator Jeong Eun-bo said South Korea regretted the US military's decision to go ahead with the furloughs with negotiations in their "final phase".

"We expect an agreement to be finalised soon," he said.

Citing an unnamed source, Yonhap news agency reported that a new deal could be announced as soon as today, "though various possibilities remain still open".

Representatives for South Korea's Foreign Ministry and the US Embassy in Seoul declined to comment on the status of the negotiations.

"We're closely watching developments," one South Korean foreign ministry official told Reuters.

The United States stations roughly 28,500 troops in South Korea, a legacy of the 1950-53 Korean War that ended in an armistice, rather than a peace treaty.

Donald Trump has repeatedly demanded that South Korea pay more, although US officials said they have since "compromised" on the initial numbers.

Abrams said USFK was working to prevent the furloughs from undermining military readiness, and said it would offer unspecified support to the workers on unpaid leave. - Reuters