Seoul's Yonhap news agency said National Intelligence Service Director Kim Sung-ho briefed parliament on the North Korean leader's state of health.
"He has slightly recovered," the spy chief was quoted as saying by Lee Chul-woo, a lawmaker for the ruling party. Lee did not give further details.
North Korean officials have denied foreign reports that the country's secretive leader has suffered a stroke, calling them attempts to harm relations between the two Koreas.
Concern over Kim's health arose when he failed to appear on September 9 at a military parade in Pyongyang marking the 60th anniversary of the communist state.
U.S. and South Korean intelligence services earlier said Kim Jong-il was in a serious condition. Both countries fear that a regime change could jeopardize the complex process of ending North Korea's nuclear program, as agreed in a six-party deal.
Under the landmark 2007 agreement between the two Koreas, China, the United States, Russia and Japan, the North pledged to dismantle its plutonium-producing Yongbyon reactor in exchange for fuel aid and other incentives. Deconstruction work began last November.
However, last week North Korea announced that preparations were underway to re-start the reactor, due to a failure by the U.S. to fulfill its side of the denuclearization deal.
The U.S. had pledged to remove North Korea from its blacklist of states sponsoring terrorism, which keeps the country in financial isolation. Washington has since said that this cannot be done until North Korea allows international inspectors to check North Korean facilities.
Kim Jong-il has ruled North Korea since 1994, when he succeeded his late father Kim Il-sung, the communist state's founder. According to Soviet records, he was born in the Russian village of Vyatskoye near Khabarovsk, where his father commanded a military brigade in which Chinese and Korean exiles served.