The allegation, if confirmed, would be the first known complete weapons system to be delivered to Russia from China for use in Ukraine.

Such a development would be of huge concern to Kyiv and its western backers and would further ratchet up tensions between Beijing and the West.

A spokesperson for the NATO alliance called the allegation "deeply concerning" and revealed that allies were "consulting on this matter".

Contacted for a response, the Chinese embassy in London said it was "not aware of the specifics you mentioned".

The embassy said: "China exercises strict export control over drones, and always strictly controls military and dual-use drones in accordance with laws and regulations.

"China's position and practice have been consistent, in stark contrast to some countries' double standards on arms sales and their behaviours to add fuel to the flames of the Ukrainian crisis.

"China has always maintained an objective and fair position on the Ukrainian crisis, actively advocated peace and promoted talks, and is committed to promoting a political solution to the crisis."

The UK and other allies have previously accused China of providing so-called dual-use products to Russia that could be used for military purposes.

These include semiconductor chips, radars and sensors that are needed when manufacturing a range of lethal weapons.

The claim about a programme in China to deliver attack drones for Russia's war in Ukraine was first reported by the Reuters news agency.

Responding to that report, a White House spokesperson expressed concern but said the US had not seen anything to suggest Beijing was aware of the alleged transactions or that there had been any Chinese government involvement in providing lethal aid to Russia.

The western official told Sky News that the Reuters reporting was accurate.

The official alleged that in 2023 a Chinese company had sent a range of purpose built military drones to Russia for testing, with the ultimate destination being Ukraine.

"There is clear evidence now that Chinese companies are supplying Russia with deadly weapons for use in Ukraine," the official said.

When asked, the official confirmed they were referring to military drones.

"While the Chinese government might not admit it, they are going to struggle to keep their increasing support under wraps," the official added.

Sky News has not seen the evidence the source referred to.

On Wednesday, Reuters reported that Russia had established a weapons programme in China to develop and produce long-range attack drones for use in the war in Ukraine, according to two sources from a European intelligence agency and documents reviewed by the news agency.

It alleged that IEMZ Kupol, a subsidiary of Russian state-owned arms company Almaz-Antey, had developed and flight-tested a new drone model called Garpiya-3 (G3) in China with the help of local Chinese specialists.

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Kupol was placed under US sanctions in December 2023.

The names of the Chinese firms allegedly involved in the programme were not revealed.

The G3 drone can reportedly travel around 1,200 miles with a payload of 50 kg (110 pounds).

Samples of the G3 and some other drone models made in China have been delivered to Kupol in Russia for further testing, again with the involvement of Chinese experts, it was alleged.

Asked about the claims, Farah Dakhlallah, the NATO spokesperson, said: "These reports are deeply concerning and allies are consulting on this matter.

"China has become a decisive enabler of Russia's war in Ukraine through its provision of dual-use technologies to sustain Russia's defence industry. The Chinese government has a responsibility to ensure its companies are not providing lethal assistance to Russia.

"China cannot continue to fuel the largest conflict in Europe since the Second World War without this impacting its interests and reputation."