North Korea's third nuclear test is expected to push Beijing into getting tougher with its wayward neighbour and reassess how it deals with it.
The Foreign Ministry said China was "firmly opposed" to Pyongyang's decision to conduct the test in the face of widespread international opposition.
It said Pyongyang should honour its commitment to drop its nuclear programme and "refrain from any move that may further worsen the situation".
The ministry did not say what, if any, specific action Beijing would take, but called for parties involved to remain "cool-headed" and resolve the issue within the framework of the long-stalled Six-Party Talks, which also include Japan, South Korea, Russia and the United States.
Beijing has long been Pyongyang's chief ally, with China accounting for almost all of North Korea's foreign trade and aid.
But the third nuclear test serves as a reminder that Pyongyang has paid no regard to pressure from China, triggering calls for Beijing to change its tactics in dealing with its secretive neighbour.
China has traditionally protected Pyongyang from international sanctions, saying they would trigger instability in the Korean Peninsula.
But last month it supported sanctions imposed by a UN resolution.
Cai Jian, a professor of Korean studies at Fudan University, said he expected Beijing would co-operate with the US, and might cut its aid to North Korea.
"China wants to show that it is not only verbally supporting sanctions, but taking concrete actions," he said.
Sunny Lee, a South Korean fellow at the Centre for Strategic and International Studies in Washington, said China's influence on North Korea appeared to be waning.
Pyongyang had a "mistrust" of Beijing, which was getting closer to Seoul, Lee said.
China supports Pyongyang in part because North Korea serves as a buffer zone that keeps US forces from China's border and gives it more influence in the region.
"Some still believe in the buffer zone theory," Peking University international relations professor Jia Qingguo said, "even though with long-range missiles and advanced technologies, China does not really need a buffer zone."
Professor Su Hao, from China Foreign Affairs University, said North Korea was "well aware" of Beijing's concern that an isolated Pyongyang might resort to more radical action.
"Beijing wants to keep normal ties with Pyongyang, and such an attitude will not be affected by the nuclear test," Su said.