Hill, the assistant secretary of state, returned to Seoul after a three-day visit with North Korean officials to meet with Kim Sook and Akitaka Saiki, his counterparts in South Korea and Japan.
Hill had traveled overland to Pyongyang Wednesday to meet with North Korean's Kim Kye-gwan and other officials on a mission to stop the country from reviving its atomic weapons drive.
"The discussions in Pyongyang were quite substantive, we went into great detail on things," Hill said in a report from Voice of America.
"They were quite lengthy ... needless to say there's been a lot of problems in the six-party process, so indeed we did quite a substantial review of activities in the last couple months."
Tensions have mounted since North Korea moved to resume its nuclear activity in recent weeks in an apparent protest against the United States' refusal to take it off the terrorism sanctions list.
Hill heads for Beijing Saturday to meet with Chinese Vice Foreign Minister Wu Dawei, who heads the talks with North Korea.