Officials and experts attribute the change in China's stance to a growing frustration with Pyongyang.
"The Chinese always have us believe their influence on North Korea is less than we think," one senior official said.
"We usually don't believe them, but the North is not listening to the Chinese now and that really frustrates them. They are having a tough time."
China's more hard-line approach to its troublesome ally has significant implications, said Kurt Campbell, who recently stepped down as the State Department's top diplomat for Asia.
"There is a subtle shift in Chinese foreign policy over the short- to medium term, that has the potential to affect the calculus in northeast Asia," Campbell said Friday at a forum at Johns Hopkins University. The Chinese, he said, are starting to recognize that North Korea's recent actions are "antithetical to their own national security interests."