New satellite images reveal ongoing activity at North Korea's atomic test site, according to a US 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 US-Korea Institute at Johns Hopkins University posted on its 38 North website Sunday. "Snowfall and subsequent clearing operations as well as tracks in the snow reveal ongoing activity at buildings and on roadways near the possible test tunnel," it said. The institute's analysis followed a series of daily threats from North Korea that it is preparing its third nuclear test in response to UN sanctions imposed on Pyongyang for a long-range rocket launch in December. In a meeting with top security officials, North Korean leader Kim Jong-UN ordered "substantial and high-profile important state measures," fuelling speculation of an imminent detonation. Military chief Hyon Yong-Chol, the head of the army's politburo Choe Ryong-Hae and spy chief Kim Won-Hong attended the meeting, state television reported Sunday. Last week's UN resolution imposing sanctions threatened further "significant action" if Pyongyang conducted a nuclear test. The North's two previous detonations in 2006 and 2009 followed a similar pattern of a missile launch, followed by UN measures, followed by a nuclear test. US warnings about the consequences of a third test have been backed by unusually strident criticism of Pyongyang by its sole major ally China. Last week, a state-run newspaper close to China's ruling Communist Party, said in an editorial that Beijing would "not hesitate" to reduce aid to North Korea if it went ahead. China has been the primary benefactor of Pyongyang since the 1950-1953 Korean War, providing critical diplomatic and economic support.