An earth tremor measured in North Korea on Tuesday morning was a "suspected explosion", China's Earthquake Administration said.
The webpage of the China Earthquake Network Centre, affiliated with the administration, said a quake "at 10:57 on February 12 in North Korea was a "suspected explosion" and occurred at a "focal depth of 0 km".
The North vowed last month to conduct a "high-level" nuclear test, its third, in retaliation for tightened UN sanctions following its December rocket launch.
China, the North's most influential ally, said last week it was "extremely concerned" by recent developments on the Korean peninsula and called on relevant parties to exercise restraint.
China has not announced whether it would implement punitive measures if North Korea conducts another nuclear test. Analysts say the emerging superpower values stability in North Korea above other foreign policy aims.