WASHINGTON, Feb 15 (Reuters) - The U.S. government on Friday called on North Korea to refrain from "additional provocative actions" following reports that Pyongyang plans more nuclear tests. State Department spokeswoman Victoria Nuland made the comment in response to a Reuters report that North Korea has told its key ally, China, that it is prepared to stage one or even two more nuclear tests this year in an effort to force the United States into diplomatic talks.

South Korea has detected an "artificial earthquake" at North Korea's nuclear test site, Yonhap news agency reported Tuesday, suggesting Pyongyang had gone ahead with a threatened atomic test. Yonhap said an "artificial earthquake of 5.1 magnitude" had been detected in Kilju county, where the Punggye-ri test site is located. The US Geological Survey measured it as a 4.9-magnitude quake at a very shallow depth of just one kilometre (0.6 miles).

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 US 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. "It will includes anti-submarine and anti-air trainings and maritime manoeuvrings," a military official was quoted as saying in the report.