To improve China’s strained relationship with the United States, Mr. Xi — who has shown himself to be more nationalistic than his predecessor — could start with getting tougher on North Korea, including at the Security Council. Advertisement Continue reading the main story 
 
 President Obama spoke Tuesday morning, hours after the test, with South Korea’s outgoing president, Lee Myung-bak, in what the White House described as an effort to agree on a common strategy at the Security Council and in the two country’s military posture on the Korean Peninsula, where troops have faced off against the North since the armistice was signed 60 years ago. Advertisement Continue reading the main story 
 
 “Moving forward, China really holds the key to what extent the actions will be different this time,” said Suzanne DiMaggio, an expert at the Asia Society. North Korea linked the test to its technical prowess in launching a long-range rocket in December, a move that triggered the U.N. sanctions, backed by China, that Pyongyang said prompted it to take Tuesday's action. The U.S. president will likely have to tweak his State of the Union address due to be given on Tuesday. But it called on all sides to "respond calmly" and said issues should be resolved within the framework of long-stalled denuclearization talks involving North Korea, China, the U.S., South Korea, Japan and Russia. Advertisement Continue reading the main story 
 
 A statement issued by the White House after the conversation included a rare reference to American’s “nuclear umbrella” over South Korea, a reminder to the North that an attack on South Korea would be viewed as an attack on the United States.
– In open defiance of the UN, North Korea has executed its third nuclear test, prompting global condemnation among leaders from the US to Russia to Japan. The latest test involved a "miniaturized and lighter nuclear device with greater explosive force than previously" in 2006 and 2009, the country said. Calling the test a "highly provocative act," President Obama has urged "swift and credible action by the international community," and the UN Security Council is planning a meeting this morning to address the issue, the New York Times reports. Pyongyang said afterward that the test was a "first response" to US hostility and warned of unspecified "second and third measures of greater intensity." The test has also angered China, Pyongyang's one major ally, Reuters notes. It's "hugely insulting to China, which now can be expected to follow through with threats to impose sanctions," says an analyst. Thus far, China has cited "firm opposition" to a test conducted "heedless of widespread international opposition," the AP reports. According to South Korea, seismic activity suggests the explosion measured some 6 to 7 kilotons—more than previous tests but less than the 20 kilotons detonated in Hiroshima. Seismic tests by several countries suggested the test, which the North called "safe and perfect," occurred near the location of its predecessors; the US Geological Survey says it was just a kilometer underground. Over at the Washington Post, the crack staff has found the exact test site—on Google Maps, just off "Nuclear Test Rd."