The United States on Tuesday announced the two-year extension of its 40-year-old accord with South Korea on civilian nuclear cooperation.

The widely anticipated announcement also heralds the start of another round of tough negotiations on Seoul's pursuit of advance consent for uranium enrichment and spent fuel reprocessing.

The State Department said the allies concluded an agreement extending the duration of the existing bilateral agreement on a peaceful nuclear partnership until March 19, 2016. It entered into force immediately.

The extension will allow the two nations to continue their "extensive and long-standing bilateral cooperation on the peaceful uses of nuclear energy in an environmentally responsible manner," the department said.

It will also provide additional time for the two sides to complete negotiations on a successor nuclear cooperation agreement, added the department.

Under the existing accord, which sets the rules for the supply of nuclear fuel and related technology, South Korea agreed not to enrich uranium and reprocess spent fuel.

South Korea hopes to expand its non-military nuclear program to meet its growing domestic demand and its status as a nuclear plant exporter.

The Barack Obama administration has been tepid on Seoul's request, however, due to concerns about a possible negative impact on its global nonproliferation drive. (Yonhap)