The United States will not agree to South Korea's reprocessing of spent nuclear fuel for commercial purposes due partly to proliferation concerns, a former U.S. State Department adviser on arms control said Friday.

Seoul and Washington are in negotiations to renew their bilateral pact, dubbed the 123 agreement, with the latest and eighth round of negotiations held in Washington earlier this week.

The agreement, last revised in 1974, was to expire in 2014, but the two countries agreed earlier this year to extend it by two years in order to buy more time amid wide opinion gaps. Seoul seeks to win rights to reprocess spent nuclear fuel and enrich uranium for commercial purposes, but Washington has repeatedly resisted the calls.

"The Republic of Korea wants the successor 123 agreement to provide U.S. advanced consent for South Korea to reprocess U.S.-origin spent fuel and enrich U.S.-supplied uranium," Robert Einhorn, a former special adviser on nonproliferation and arms control at the State Department, said through a statement before a news conference in Seoul.

"The U.S. doesn't want to provide that consent now. Instead, it wants to keep the issue under review and defer a decision on consent until later," said the former U.S. nonproliferation czar.

Seoul seeks the consent mainly in order to alleviate the spent fuel storage problems, ensure reliable access to enriched uranium to fuel domestically manufactured nuclear reactors as well as to promote competitiveness of the local nuclear industry, the former official said, citing South Korean President Park Geun-hye's remarks during her visit to Washington earlier this year.

"The U.S. believes they can be achieved without taking a decision at the present time to provide advanced consent for pyroprocessing and enrichment in the ROK," said Einhorn, who is now a senior fellow at the Brookings Institution.

The decision should be deferred until results come out of the 10-year Joint Fuel Cycle Study project between Seoul and Washington, which kicked off in 2011, he said.

Proliferation concerns over nuclear-arms seeking countries including North Korea are also the reason why the U.S. is not willing to provide the consent, Einhorn also noted.

"The problem is the example and precedent it sets for others.

More and more countries are now interested in nuclear energy, among them Saudi Arabia, Jordan and Vietnam. If a successor agreement provides advanced consent, it will send a signal that the U.S. is comfortable with the spread of indigenous fuel cycle capabilities," he said, adding that "this will be damaging to our shared nonproliferation goals."

Seoul and Washington's efforts to denuclearize North Korea will face a greater challenge if the South is allowed to have its own fuel cycle programs, he said. "We know that this difficult task will be made even more difficult if the North Koreans can cite active fuel cycle programs in the South."

Mindful of public criticism in South Korea over U.S.' reservations, Einhorn called on the allies for early agreement saying, "It could elevate the issue to the top of the bilateral agenda, become a major irritant, lead to a hardening of public opinion on both sides and put our presidents in a difficult position."

The nonproliferation expert said he believes North Korea has restarted its plutonium reactor at its Yongbyon nuclear facility.

"I think there is a number of indications that they have restarted their five megawatt reactor. I think the evidence is pretty strong that they have restarted it," he said after the news conference. (Yonhap News)