WASHINGTON (Yonhap) ― South Korea and the United States are in the final stage of negotiations on a civilian nuclear cooperation deal, but it is unclear whether the talks will be concluded before year’s end, a senior diplomatic source indicated Monday.

After more than four years of negotiations, Seoul and Washington now appear close to an agreement on how to revise their 1974 nuclear energy cooperation pact, known as the “123 agreement,” which bans the South from enriching uranium or reprocessing spent fuel in exchange for U.S. technological assistance.

But Seoul has demanded that it be allowed to undertake enrichment and reprocessing activities, just as Japan is allowed to under a U.S. bilateral agreement. Washington has been reluctant about Seoul’s demand due to proliferation concerns.

Last month, the two sides said in a joint statement following their “two plus two” alliance talks that they had made “significant progress” in the sensitive negotiations. That spurred widespread views that a deal would be possible before the end of the year.

South Korean officials have long said that they consider the year’s end to be the deadline for the negotiations, considering the time needed for legislative approval and other procedures necessary in both countries for putting the envisioned agreement into effect.

“As we’ve seen in other negotiations, even if we make a lot of headway, we have to sort out some final tasks and resolving them is not necessarily easy,” the source said on condition of anonymity.

“I think we’re at that stage with regard to the atomic energy pact.”

Though there is not much time left for the negotiations, making a “balanced and future-oriented” agreement is more important than concluding a deal early, he said. The remarks suggest that the two sides may miss the year-end deadline.

Media speculation has been rife about the scope of the final agreement.

One possibility is that the U.S. could allow the South to conduct reprocessing only for research purposes while leaving other reprocessing and enrichment activities subject to advance consent from the U.S.

The 1974 pact had been scheduled to expire in March this year, and Seoul and Washington launched negotiations in 2010 to revise it. But after failing to find a compromise, they extended the existing pact to March 2016 to buy more time for negotiations.