Trump, who has emerged as the undisputed frontrunner for the Republican presidential nomination in the 2024 election, had during his presidency accused key Asian ally South Korea of "free-riding" on U.S. military might and demanded it pay as much as $5 billion a year for the U.S. deployment.

Negotiations for the Special Measures Agreement were gridlocked for months under the Trump administration, and the deal was finalised when South Korea agreed to a 13.9% increase in its contribution, the biggest annual rise in nearly two decades.

The agreement is set to expire in 2025 and Yonhap news agency as well as news service Newspim quoted unnamed diplomatic sources as saying South Korea and the United States had agreed to start talks this year on extending the deal to 2026 and beyond. Talks are usually held just before the existing deal is due to end.

South Korean Foreign Ministry spokesman Lim Soo-suk declined to comment on the reports, saying the government would prepare for the next negotiations in a "systematic, strategic" manner.

The U.S. State Department did not immediately respond to a request for comment.

American troops are deployed in South Korea as part of both nations' efforts to deter North Korea, which has been accelerating its nuclear and missile programmes.

South Korea began shouldering the costs of U.S. deployments, used to fund local labour, the construction of military installations and other logistics support, in the early 1990s.

(Reporting by Hyonhee Shin; editing by Miral Fahmy)

By Hyonhee Shin