North Korea, China's longtime ally, has vexed Beijing for years with its rocket launches, nuclear tests, kidnapping of Chinese fishermen and other erratic behavior. Yet, Beijing has run interference at the United Nations to temper punishments against Pyongyang, and has even helped Pyongyang circumvent sanctions. China is trying to punish ally North Korea for its nuclear and missile tests, stepping up inspections of North Korean-bound cargo in a calibrated effort to send a message of Chinese pique without further provoking a testy Pyongyang government. North Korea's economic lifeline, China is showing signs of getting tough with an impoverished neighbor it has long supported with trade, aid and diplomatic protection for fear of setting off a collapse. The moves to crimp, but not cut off trade with North Korea come as Beijing falls under increased scrutiny to enforce new U.N. sanctions passed after last month's nuclear test, Pyongyang's third. Targeted in the sanctions are the bank financing and bulk smuggling of cash that could assist North Korea's nuclear and missile programs as well as the luxury goods that sustain the ruling elite around leader Kim Jong Un. Pyongyang has reacted with fury and threatening rhetoric against South Korea and the U.S. China's change of tack with North Korea unlikely foreshadows a total end to Beijing's support. For Beijing, North Korea remains a pivotal strategic buffer between China and a U.S.-allied South Korea, and Chinese leaders worry that too much pressure could upend an already fragile North Korean economy and cause the Kim government to collapse, leaving Beijing with a security headache and possible refugee crisis.