Satellite images have emerged of a top secret base in China that has comparisons to the highly classified Area 51 in the US.
The images show new buildings around an isolated airstrip in the desert in China's Xinjiang province amid a race between China and Western countries to develop secret weapons.
China has been rapidly building up its nuclear forces, causing concern to the US who called on Beijing to engage with it "on practical measures to reduce the risks of destabilising arms races".
The concern comes after The Washington Post reported China had begun constructing more than 100 new missile silos in a desert earlier this month.
Like Area 51 in the US state of Nevada, the satellite images taken of a site hundreds of kilometres from civilisation show dozens of buildings constructed in the desert near the airstrip that did not exist a year ago.
Speculation over mysterious Chinese site
The images, provided to National Public Radio by space technology company Maxar, show the mysterious new site is also close to China's former nuclear weapons testing site at Lop Nur.
Speculation over the new constructions has also been sparked after a highly classified space plane landed at the airstrip last year.
Senior fellow at the Carnegie Endowment for International Peace, Ankit Panda, told NPR the new buildings at the airstrip could be supporting China's military space activities.
"I think we're observing what appears to be a pretty important facility for China's military space activities that appears to be growing," he said.
With the airstrip mostly dormant in recent years, experts suggest the emergence of new structures could be a sign of more planned activity.
"This seems to be something that is more than just, 'We're coming here for the weekend'," Centre for Astrophysics Harvard and Smithsonian astronomer Jonathan McDowell told NPR.
'The build-up is concerning'
Earlier this month The Washington Post reported China was constructing silos for intercontinental ballistic missiles near the city of Yumen.
US State Department spokesperson Ned Price told a regular news briefing it was concerning as it could expand China's nuclear capabilities.
"These reports and other developments suggest that the PRC's nuclear arsenal will grow more quickly, and to a higher level than perhaps previously anticipated," Price said using the acronym for the People's Republic of China.
"This build-up is concerning. It raises questions about the PRC's intent. And for us, it reinforces the importance of pursuing practical measures to reduce nuclear risks.
"We encourage Beijing to engage with us on practical measures to reduce the risks of destabilising arms races – potentially destabilising tensions."
Washington has repeatedly called on China to join it and Russia in a new arms control treaty and the US disarmament ambassador said in May that Beijing was resisting this despite a "dramatic" build-up in its arsenal.
Beijing says its arsenal is dwarfed by those of the United States and Russia and it is ready to conduct bilateral dialogues on strategic security "on the basis of equality and mutual respect".
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