South Korea and the United States decided to extend formal negotiations aimed at revising a bilateral civilian nuclear accord for an additional day, a Seoul official said Thursday, after two days of talks failed to narrow differences.

The allies began a new round of talks on Tuesday in Washington to rewrite the 1974 agreement that bans Seoul from reprocessing spent fuel because it could yield plutonium that could be used to build atomic bombs.

Seoul wants Washington to allow it to use a proliferation-resistant technology for enriching uranium and reprocessing spent atomic fuel, but Washington has been reluctant to do so apparently because of proliferation concerns.

“Negotiators from the two nations decided to continue the talks on Thursday (Washington time),” Seoul’s Foreign Ministry spokesman Cho Tai-young told reporters.

Cho said that South Korea and the U.S. have been engaged in “in-depth” negotiations, but declined to go into details.

Earlier in the day, a government source in Seoul said the two sides have failed to narrow differences during this week’s talks.

The source, who spoke on the condition of anonymity, also hinted that this week’s talks may be open-ended, saying, “We will know when the talks will be ended once the two sides meet on Thursday.”

For a revised accord to be approved by the U.S. Congress, both sides must conclude negotiations by this summer, officials said.

South Korea, a major nuclear energy developer, wants the U.S. to allow it to adopt proliferation-resistant technology for enriching uranium and reprocessing spent atomic fuel from its 23 nuclear power plants.

In the face of growing nuclear waste stockpiles and its ambition to become a global power in the civilian nuclear industry, South Korea hopes to adopt the so-called pyroprocessing technology, which leaves separated plutonium, the main ingredient in making atomic bombs, mixed with other elements.

South Korea wants the U.S. to allow it to use the new technology because it has to deal with more than 10,000 tons of nuclear waste at storage facilities that are expected to reach capacity by 2016.

Some nonproliferation experts say pyroprocessing is not significantly different from reprocessing, and the plutonium could be quickly turned into weapons-grade material. (Yonhap News)