BEIJING China's tight bond with North Korea faced mounting pressure with the arrival on Tuesday of a U.S. envoy as Washington urges Beijing to use its influence over Pyongyang to curb its nuclear weapons ambitions.
The impoverished and isolated North depends heavily on its only major ally for economic and diplomatic support and its leader Kim Jong-il has visited China twice this year, in part to gain backing for the anointment of his son to eventually take over the family dynasty.
Those ties have become a sore-point with Washington after revelations that North Korea appears to have made big steps toward enriching uranium -- one pathway to making the fissile material needed for nuclear weapons -- possibly using technology that passed through or even originated in China.
A U.S. academic, Siegfried Hecker, who recently visited North Korea, said at the weekend that he had seen more than a thousand centrifuges for enriching uranium during a tour of the North's Yongbyon nuclear complex.
Sung Kim, a U.S. official dealing with North Korean issues, said in Washington D.C. on Monday that China's ties its chairing of stalled talks aimed at ending Pyongyang's nuclear weapons capability mean "they do have a special responsibility to deal with the challenges posed by North Korea."
The top U.S. envoy for the North Korean nuclear dispute, Stephen Bosworth, was due to travel from Tokyo to Beijing on Tuesday.
"Needless to say, we regard this development with great seriousness. We do not consider it a crisis but it is a very serious development," Bosworth told reporters in Tokyo.
"We believe that the North Koreans are in violation of a substantial number of international agreements that they have entered into and are in violation of U.N. council resolutions," he said.
North Korea has said it wants to restart six-party nuclear disarmament talks it abandoned two years ago. But Seoul and Washington have said the North must move forward with previous pledges to curtail its nuclear program.
China has urged returning to the nuclear disarmament negotiations but has also fended off calls from the U.S. and its regional allies to use it's vital food and energy aid to North Korea as a lever.
Beijing says Washington should be more flexible.
In recent years, Beijing has sought to strengthen relations and increased aid and investment to its poor neighbor, which it sees as a strategic buffer against the U.S. and its regional allies.
In early May, Kim Jong-il visited China on his first trip abroad since 2006. He visited again in late August, ahead of the emergence of his son, Kim Jong-un, as the favoured successor in the dynastic one-party regime.
"With China accelerating the scale and scope of their bilateral capacity-building activities in North Korea, it's difficult to see any sense of urgency in Pyongyang to resume diplomacy with Washington," said John Park of the U.S. Institute of Peace in Washington D.C. who studies ties between China and North Korea.
(Additional reporting by Nathan Layne, Paul Eckert and Jeremy Laurence; Editing by Jonathan Thatcher)