South Korea on Monday warned against raised expectations of a major breakthrough in relations with North Korea, saying Pyongyang had some way to go before sanctions could be eased.

The two Koreas agreed to resume a high-level dialogue during a surprise and extremely rare visit to the South on Saturday by a top-ranking delegation from Pyongyang led by Hwang Pyong-So -- widely seen as the number two to paramount leader Kim Jong-Un.

There is nothing wrong with the health of North Korean leader Kim Jong-Un, a top official said during a rare visit to the South, after speculation of a debilitating illness and even wild rumors of a coup in Pyongyang.

Kim has not been seen in public for more than a month -- an unexplained absence that loomed large over the surprise visit on Saturday by a trio of his top officials and closest aides.

Recent satellite images suggest North Korea may have shut down the nuclear reactor seen as its main source of weapons-grade plutonium, a U.S. think-tank said Saturday.

The Washington-based Institute for Science and International Security (ISIS) said any shutdown could be for either partial refueling or renovation.

North and South Korea agreed Saturday to work on resuming a formal high-level dialogue that has effectively been suspended for seven months, the South's Unification Ministry said.

The agreement came during a surprise visit to South Korea by three top-ranking North Korean officials, two of them close aides to paramount leader Kim Jong-Un.

One of three Americans detained in North Korea has made a fresh plea for his government's help ahead of a trial that will likely end with a lengthy prison term and hard labor.

In an interview published Thursday by the pro- North Korean Japanese newspaper Chosun Sinbo, Jeffrey Fowle said he was extremely "anxious" that he would share the fate of his already tried and jailed fellow detainees -- Kenneth Bae and Matthew Miller.

Fresh satellite images show North Korea has completed upgrading its main satellite site to handle far larger rockets, suggesting a possible launch by the end of the year, a U.S. think tank said Thursday.

North Korean leader Kim Jong-Un, who has not been seen for nearly a month due to apparent health problems, sent a congratulatory message to China on its national day Wednesday, state media said.

Japan on Monday demanded North Korea promptly report the results of its probe into the Cold War kidnappings of Japanese citizens as the two nations began talks in China.

The meeting of senior officials in Shenyang came after Tokyo announced in July it was easing sanctions against Pyongyang, following the secretive state's promise to reinvestigate the cases of Japanese abductees.

North Korea's foreign minister on Saturday berated the U.N. Security Council as a forum of lies and double standards, in the first U.N. address by a top Pyongyang official in 15 years.

Foreign Minister Ri Su Yong called for reforming the U.N. Security Council, which has imposed tough sanctions on North Korea over its suspect nuclear program and missile launches.

North Korean state media has confirmed that leader Kim Jong-Un is suffering "discomfort" in its first comment on health issues believed to be behind his three week absence from public view.

In an hour-long documentary entitled "Days of guidance for the people" that was aired Thursday, state television showed a limping Kim visiting a factory outside Pyongyang, South Korea's Yonhap news agency reported Friday.