"At the start, China gave him a warm welcome and, I think, some aid. But we got no gratitude. They take us for granted," said Jin Canrong, an international affairs expert at Renmin University in Beijing. "China tried to get closer to him, but it was not successful. China has become very disappointed."Yet Beijing also sees Pyongyang as a crucial buffer against U.S. troops based in South Korea and Japan. It also deeply fears a regime collapse could send swarms of refugees across its border. For those reasons, Beijing is unlikely to cut Pyongyang adrift, even if it pushes North Korea harder to end its nuclear provocations and reform its broken-down economy. "China's not ready to turn the support to North Korea switch to 'off' at this stage," said Roger Cavazos, a North Korea watcher at the Nautilus Institute for Security and Sustainability. North Korea's apparent reluctance to reform its economy ranks among Beijing's biggest frustrations, and the thorny nature of the bilateral relationship is on show along the frigid Yalu River, which forms part of the border Chinese troops crossed to rescue North Korean forces during the 1950-53 Korean War. Last week, ahead of the Lunar New Year holiday, dozens of North Korean trucks lined up at a customs checkpoint in the northeastern Chinese border city of Dandong, loaded with bags of rice, cooking oil, cheap electronics and other daily items that their country's collapsed industry cannot produce enough of for its 24 million people. Further to the south, a much-heralded North Korean economic zone on a pair of islands along the Yalu remains a field of untrammeled snow behind a newly erected border fence, more than 18 months after it was opened with great fanfare. The Hwanggumphyong and Wihwa islands zone, one of two such establishments along the border, resulted from talks led by Chinese Commerce Minister Chen Deming and Jang Song Thaek, Kim's uncle and a top official in the ruling Korean Workers' Party who is thought to be pro-China. The two men attended the June 2011 grand opening accompanied by a brass band and the celebratory release of doves, giving rise to hopes that China's advice was having an impact on the North.