The U.S. State Department confirmed Tuesday that it is replacing Washington's top negotiator in the thorny talks with South Korea on their civilian nuclear cooperation.

"We can confirm that Special Advisor Robert Einhorn is leaving the State Department," a department official told Yonhap News Agency, requesting anonymity.

Over the past two years, Einhorn, the department's nonproliferation czar, has led U.S. negotiations with South Korea on revising their accord on nuclear energy.

"Assistant Secretary of State for International Security and Nonproliferation Thomas Countryman will continue our negotiations with the Republic of Korea on a successor civilian nuclear cooperation agreement," the official said.

Countryman plans to lead a U.S. delegation to Seoul in June for further discussions, added the official.

In April, South Korea and the U.S. agreed to extend the current agreement, originally set to expire in March next year, until 2016.

The deal was apparently aimed at buying time for more talks after failing to narrow differences on Seoul's pursuit of uranium enrichment and reprocessing.

Under the existing accord, signed in 1974 for the transfer of nuclear technology and materials from the U.S., South Korea can't be involved in those activities even for peaceful purposes.

South Korea has argued that it needs to expand its nuclear activities to meet growing domestic demand for nuclear energy and Seoul's enhanced status as a nuclear plant exporter.

But U.S. officials remain concerned about the possible negative impact on their global nonproliferation campaign.

Diplomatic sources here said the change of top U.S. negotiator would not mean an immediate shift in Washington's position, especially as Countryman has been handling nonproliferation issues at the department.

Einhorn is expected to work at the Brookings Institution, according to diplomatic sources.

Meanwhile, the allies are set to open a new round of talks next month on sharing the cost of stationing 28,500 American troops in Korea, said the sources.

"Amb. Eric G. John, foreign policy adviser to the Air Force chief of staff, has been named as Washington's top delegate," a source said. "The two sides are expected to start negotiations in late June."

John, a career diplomat, primarily worked in East Asia, having served as minister-counselor for political affairs at the U.S. Embassy in Seoul.

His South Korean counterpart is Hwang Joon-kook, former deputy chief of mission of the South Korean Embassy in Washington.

The U.S. is reportedly asking South Korea to increase its financial support for the U.S. Forces Korea as Washington is slashing its defense budget. (Yonhap News)