WASHINGTON: China is backing Russia's war effort in Ukraine by helping Moscow in its biggest military buildup since the Soviet era, providing drone and missile technology, satellite imagery and machine tools, senior U.S. officials said on Friday.

However, the Chinese Embassy in the U.S. said it has not provided weaponry to any party, adding that it is "not a producer of or party involved in the Ukraine crisis."

The officials, speaking on condition of anonymity, said U.S. President Joe Biden raised the issue with Chinese President Xi Jinping in their recent phone call and that it is a topic of discussion with U.S. allies in Europe and around the world.

One official said Chinese materials are filing critical gaps in Russia's defense production cycle and helping Moscow undertake its "most ambitious defense expansion since the Soviet era and on a faster timeline than we believed possible early on in this conflict."

"Our view is that one of the most game-changing moves available to us at this time to support Ukraine is to persuade the PRC (China) to stop helping Russia reconstitute its military industrial base. Russia would struggle to sustain its war effort without PRC input," the official said.

A Chinese embassy spokesperson told Reuters that normal trade between China and Russia should not be interfered or restricted.

"We urge the U.S. side to refrain from disparaging and scapegoating the normal relationship between China and Russia," Liu Pengyu said.

Some of the information provided by the U.S. officials in a small briefing with reporters was based on declassified intelligence. They sketched a wide array of ways China is helping Russia's two-year war against Ukraine without providing lethal assistance.

President Joe Biden has been pressuring Republicans who control the U.S. House of Representatives to approve a major infusion of funding for providing weapons to Ukraine as it struggles to fend off the Russians.

The U.S. and its allies have also been more blunt about confronting Chinese aggression in the South China Sea and against self-ruled Taiwan.

China complained about what it viewed as anti-China rhetoric emanating from Biden's talks this week with the leaders of Japan and the Philippines, prompting a denial from the White House.

The Russians have likely used machine tool imports from China to increase its ballistic missile production, the officials said. They cited Dalian Machine Tool Group, one of China's leading machine tool manufacturers, as one company supplying Russia.

In 2023, 90 per cent of Russia's microelectronics imports came from China, which Russia has used to produce missiles, tanks and aircraft, the officials said.

They said that Chinese companies such as Wuhan Global Sensor Technology Co, Wuhan Tongsheng Technology Co Ltd and Hikvision are providing Chinese optical components for use in Russian systems, including tanks and armored vehicles.

In addition, Russia has received military optics for tanks and armored vehicles that Chinese firms iRay Technology and North China Research Institute of Electro-Optics manufacture, they said.

The officials also said China has provided Russia with drone engines and turbojet engines for cruise missiles, and that Chinese and Russian entities have been working to jointly produce drones inside Russia.

Chinese companies are likely providing Russia with nitrocellulose to make propellants for weapons, helping Russia rapidly expand its capacity to make key munitions like artillery rounds, they added.

The U.S. officials also said China is helping Russia improve its satellite and other space-based capabilities for use in Ukraine, increasing the Russian threat across Europe. And they said the U.S. has information China is providing imagery to Russia for its war on Ukraine.

 -Reuters