"North Korea asked us to focus on the issue of unused fuel rods," South Korea's deputy chief nuclear envoy Hwang Joon-kook told reporters after returning from a five-day visit fact-finding mission to decide whether to buy the rods.
Seoul said earlier it would consider buying the rods if they could be used at its civilian nuclear power plants.
Yonhap also quoted Hwang as saying his discussions with North Korean officials in Pyongyang were confined to "technical aspects," and that he "was limited in who he could meet."
He also said he was not allowed to visit the North's Foreign Ministry or meet its chief nuclear negotiator, Kim Kye-gwan.
The visit was part of talks involving six countries - the two Koreas, the U.S., Russia, China and Japan - on a deal to provide fuel and economic incentives to North Korea in exchange for Pyongyang giving details of its nuclear program and disabling its atomic facilities. The process recently came to a halt amid diplomatic wrangling.
"We looked around the nuclear facilities in Yongbyon that are being disabled," Hwang said, adding that his team had confirmed that about 14,800 unused fuel rods, which are equivalent to 101.9 tons of uranium, were being stored at the Yongbyon complex. The materials are reportedly worth over US$10 million.