South Korea and the United States will hold a joint naval exercise next week, a report said Saturday, a move seen as a warning to North Korea ahead of its widely expected nuclear test.

The three-day exercise involving a U.S. nuclear submarine and other warships will begin on Monday in the Sea of Japan (East Sea) off the South Korean port city of Pohang, Yonhap news agency reported.

South Korean President Lee Myung-Bak held an emergency security meeting Thursday that warned North Korea of "serious consequences" if it went through with an expected nuclear test.

The meeting at the presidential Blue House included the defense minister, National Intelligence Service chief and the national security adviser.

North Korea on Tuesday vowed "merciless" retaliation against the South for its support of U.N. sanctions, as Seoul urged Pyongyang to step back from a widely expected nuclear test.

The perennially tense situation on the Korean peninsula has been stretched to its limit in the past week, with almost daily threats from the North that it is preparing to conduct a nuclear test as a riposte to the expanded sanctions.

Weeks after its chairman Eric Schmidt's secretive visit to North Korea, Google has rolled out a detailed map of the isolated state that even labels some of its remote and infamous gulags.

Until now North Korea was pretty much a blank canvas to users of Google's "Map Maker", which creates maps from data that is provided by the public and fact-checked in a similar process to that used by Wikipedia.

The top U.S. envoy on North Korea dismissed hopes for an immediate diplomatic solution over Pyongyang's expected nuclear test, saying Monday that the isolated state was "bent on playing a game of risk".

The comments from Glyn Davies come as North Korea threatens daily that it is preparing its third nuclear test in response to U.N. sanctions imposed on Pyongyang for a long-range rocket launch in December.

New satellite images reveal ongoing activity at North Korea's atomic test site, according to a U.S. research institute, as expectation mounts of an imminent nuclear detonation by the isolated state.

The images, as recent as January 23, suggest the facility would be ready to conduct a test "in a few weeks or less" once the order is given, the U.S.-Korea Institute at Johns Hopkins University posted on its 38 North website Sunday.

Japan Sunday launched two satellites to strengthen its surveillance capabilities, including keeping a closer eye on North Korea which has vowed to stage another nuclear test.

One of them was a radar-equipped unit to complete a system of surveillance satellites that will allow Tokyo to monitor any place in the world at least once a day.

North Korea has vowed "high-profile measures" in the latest in a series of threats sparked by a tightening of U.N. sanctions, state media said Sunday, suggesting it was determined to press ahead with a third nuclear test.

The warning by leader Kim Jong-Un came a day after Pyongyang said the planned test was a "demand of the people" following sanctions adopted last week in response to the North's defiant long-range rocket launch on December 12.

North Korea Saturday renewed its threat to carry out a third nuclear test in the latest in a series of bellicose warnings sparked by a tightening of U.N. sanctions, saying it was the "demand of the people."

It came a day after Pyongyang said the sanctions adopted earlier this week amounted to a "declaration of war", threatening the South with unspecified "physical counter-measures".

North Korea's sole major ally China will decrease aid to Pyongyang if it goes ahead with a planned nuclear test, state-run media said in an unusually frank warning on Friday.

China is the North's leading energy supplier and trade partner and is seen as one of the few nations able to influence Pyongyang's behavior, with the comments adding a distinctive edge to its typical official calls for stability and dialogue.