North Korea congratulated China on its 19th Communist Party Congress on Wednesday amid increasingly frayed relationships between the traditional allies, as China tightens sanctions over Pyongyang's nuclear weapons programme.

The central committee of the North's ruling Workers' Party of Korea said that China had made "great progress in accomplishing the cause of building socialism with Chinese characteristics" under the correct guidance of the Communist Party.

"We are greatly pleased over this," the party central committee said in the message carried by the official KCNA news agency, adding that it "sincerely wished" the China congress "satisfactory success".

Chinese Foreign Ministry spokesman Lu Kang said that many foreign political parties, organisations and dignitaries had sent their congratulations, including the Workers' Party of Korea.

"This all shows that the international community is paying close attention to the 19th Party Congress. We express our thanks for this," Lu told a daily news briefing.

Officials and experts in South Korea had worried that Pyongyang may conduct a weapons test to coincide with the opening of the twice-a-decade congress in Beijing, but there was no sign of that as Chinese President Xi Jinping gave his opening speech.

While the United States and its allies, and many people in China, believe Beijing should do more to rein in Pyongyang, the acceleration of North Korea's nuclear and missile capabilities has coincided with a near-total breakdown of high-level diplomacy between the two.

China, Pyongyang's sole major ally and which accounts for more than 90 percent of world trade with the isolated country, has said it will strictly enforce U.N. Security Council sanctions banning imports of coal, textiles and seafood, while cutting off oil shipments to the North.