North Korea, which regularly threatens the South and the United States with destruction, is already under heavy sanctions imposed by several UN resolutions beginning in 2006 but has defied pressure to abandon its missile and nuclear programs.
It last conducted a nuclear test in February 2013.
"China has told North Korea that there is no justification for a new nuclear test and that they should not do it," said a Western diplomat who was briefed by Chinese officials.
The sources said China had used diplomatic channels in Beijing and Pyongyang to convey its anxiety about the possibility of a fourth test to the North.