SEOUL -- The U.S. and South Korea are at a standoff over a cost-sharing deal for the 28,500 American troops stationed on the peninsula as President Donald Trump demands Seoul foot more of the bill.

The five-year Special Measures Agreement, which outlines cost sharing for U.S. forces in South Korea, expired Monday. Negotiations began in March but an agreement could not be reached at working-level talks that started here Dec. 11.

South Korea is responsible for the salaries of South Koreans employed by the U.S. military, the construction of facilities, and such expenses as ammunition storage and aircraft maintenance. It spent 960.2 billion won ($864 million) on such aid in 2018. The U.S. has apparently requested that Seoul cover the cost of deploying new aircraft carriers and bombers.

Trump has reportedly demanded that South Korea double its contribution to about $1.6 billion per year, according to The Wall Street Journal.

Although the contract's expiration will not directly impact military exercises or combat operations, about 8,700 South Koreans who work for the U.S. forces will not receive pay. The U.S. Forces Korea command sent a notice to their labor union saying that unpaid leave will be unavoidable starting in mid-April.

The previous agreement also faced delays, with Seoul and Washington failing to wrap up negotiations by the end of 2013. But an agreement was reached quickly thereafter, in January 2014.

The deadlock is likely to drag out under Trump, who may consider shrinking the U.S. military presence in South Korea, unlike the Obama administration of five years ago. Trump could use the threat of a troop cut as a bargaining chip in future talks after Defense Secretary James Mattis, a proponent of the alliance, leaves his post as early as Monday.

South Korean President Moon Jae-in, who has pushed for rapprochement with North Korea, may accept a partial reduction of U.S. forces out of opposition to American demands to shoulder a heavier financial burden. That could weaken international pressure on Pyongyang to denuclearize.