China's propaganda authorities have issued an order barring domestic media outlets from doing original reporting on North Korean nuclear and missile issues, sources with knowledge of the situation said Sunday.

The gag order is apparently aimed at preventing the already high tensions over North Korea from escalating before and during the Chinese Communist Party's twice-a-decade congress due to open Wednesday.

After North Korea conducted its sixth and most powerful nuclear test on Sept. 3, Chinese media executives were told that news stories on North Korea must be basically in line with those of the country's official media, according to the sources who spoke on condition of anonymity.

The ruling party's powerful propaganda apparatus urged the executives to ensure that news stories on North Korea do not exacerbate public frustration and asked that stories are not covered in depth even if they are based on original information or perspectives, the sources said.

A Chinese newspaper source said publication of all its prepared stories on North Korea had to be canceled irrespective of their content after the order was handed down.

Some of North Korea's weapons tests this year coincided with key Chinese political events. Its sixth nuclear test took place just hours before the start of a summit of the so-called BRICS countries -- Brazil, Russia, India, China and South Africa -- hosted by Chinese President Xi Jinping in the city of Xiamen.

A launch of a North Korean ballistic missile in May was timed for when Xi was to open a meeting in Beijing with leaders from nearly 30 countries to promote his initiative of expanding links along and beyond ancient Silk Road trade routes.

The forthcoming party congress is China's most important political event this year, at which Xi will be formally granted a second five-year term as party general secretary, and likely given more power by filling many senior posts with trusted allies and followers, after months of intense horse trading among different factions.

For the party's and Xi's honor, China cannot allow any instability to occur before and during the congress that is expected to run for about a week.

Through its state-run media, North Korea has vented its anger at China, criticizing its longtime ally for approving U.N. sanctions against Pyongyang in support of the United States.

North Korea has called on Chinese newspapers to stop making "reckless remarks" and show a better understanding of why Pyongyang needs to advance its arms technologies and capabilities.

"Chinese media had better watch how (North Korea) smashes the hostile forces' arrogance and high-handed practices, rather than kowtowing to the ignorant acts of the Trump administration," said a lengthy article put out by the country's Korean Central News Agency in September, referring to U.S. President Donald Trump.

"They had better mind their own businesses, before impudently pointing an accusing finger at others."

North Korea has said its possession of nuclear and missile technologies is non-negotiable as it is vital for it to defend itself against perceived and overt military threats from the United States.