Hill has been in Pyongyang since Wednesday seeking a deal that would allow monitors into the secretive country to check claims it made about its nuclear program in exchange for better trading ties and standing in the international community.
Officials in Seoul said Hill had extended his stay in the North, indicating a deal might be in the works.
The nuclear agreement North Korea struck with five regional powers seemed in peril after an angry Pyongyang last month vowed to rebuild its aging Yongbyon nuclear plant in anger at not being removed from a U.S. terrorism blacklist.
The United States said it would take the North off the list once it has reached a system to verify its nuclear claims. The North started to disable Yongbyon in November under the deal.
North Korea has been sending out mixed signals over the past few weeks. It has fanned hopes of backing off its threats by inviting Hill to visit and holding its first direct talks with South Korea in almost a year.
However, South Korean government officials have told local media the North may be looking to ratchet up tensions by making moves around the site where it launches missiles and the spot where it held its first and only nuclear test two years ago.
Adding to the level of uncertainty is the health of the North's leader Kim Jong-il. U.S. and South Korean officials said he may have suffered a stroke in August, raising questions about succession in Asia's only communist dynasty and who is making the decisions regarding its nuclear program.
The North's state media has not reported on any public activities involving Kim for about 50 days. But on Thursday, its communist party newspaper referred to his health for the first time since the reports of his stroke, saying Kim had a tiring summer inspecting all parts of his country.
"While everyone else in the world is busy vacationing, we saw a flow of news sent into the universe on Kim's endless on-site inspections through a long and rough journey," it said.
