China, North Korea's closest ally, is condemning Pyongyang's announcement that it had exploded a nuclear device. News of the test came just a day after China had joined longtime rival Japan in condemning Pyongyang's plans to conduct a nuclear test.

In a statement carried by the Xinhua state news agency, the Chinese government Monday said China is "resolutely opposed" to the North Korean action. The statement, however, did not say what - if any - actions Beijing would take as a result.

China has warned the North repeatedly not to test. Shi Yinhong is an international relations professor at Beijing's People's University.

"China has already felt betrayed by its so-called ally, North Korea, previously. But this is of course, more serious than any action that North Korea has taken before," he said.

The Chinese statement Monday was unusually blunt, saying that Pyongyang had acted "brazenly". Normally, Beijing uses more moderate language about its impoverished neighbor.

Shi and other analysts say the test may push Beijing - the chief supplier of food and fuel to North Korea - to implement tougher sanctions than it has been willing to do before.