China has used diplomatic channels to warn North Korea against conducting a fourth nuclear test, multiple China-based diplomatic sources told Reuters, after the reclusive state renewed its threat of "counter-measures" against perceived U.S. hostility.

North Korea, which regularly threatens the South and the United States with destruction, is already under heavy sanctions imposed by several U.N. 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.

China is North Korea's most important diplomatic and economic ally, though three nuclear tests and several rounds of saber rattling have tested Beijing's support.

But China had not threatened the North with explicit consequences, the sources said, and its message to the North had remained consistent.

"They are against another nuclear test - but it is a mistake to believe that China is getting more severe," said a second diplomatic source, adding that China had raised the issue with a special envoy appointed by North Korean leader Kim Jong Un.

The United States has said it hopes China will use its influence to coax the North to abandon its banned nuclear weapons program. In recent public statements, Chinese foreign ministry officials have repeatedly called on all parties to "exercise restraint" on the Korean peninsula, without pointing the finger at North Korea alone.

China signed on to tougher U.N. sanctions last year after the third nuclear test, but has come under criticism from western countries and independent experts for failing to properly implement them.

China's stability-obsessed government fears the continuing development of North Korea's nuclear program will unsettle the region. Foreign Minister Wang Yi said in March that denuclearization on the peninsula was the only road to peace, and that China would not permit war or instability on its doorstep.

North Korea's official Rodong Sinmun newspaper said this month the country was justified in using all available means at its disposal to counter aggressive challenges by the United States and South Korea aimed at stifling its sovereignty.

Zhang Liangui, a North Korea expert at China's Central Party School, said it was likely China would support another round of sanctions on the North if it went ahead with another test.

"It's very possible that China would support even tougher sanctions on North Korea," he said. "I think China should also think about adjusting the aid it provides.

