A military base deep inside Saudi Arabia appears to be testing and possibly manufacturing ballistic missiles, experts and satellite images suggest, evidence of the type of weapons programme it has long criticised its arch-rival Iran for possessing.
Saudi's powerful Crown Prince Mohammed bin Salman raised the stakes for such a programme when he said last year that the kingdom would not hesitate to develop nuclear weapons if Iran does. Ballistic missiles can carry nuclear warheads to targets thousands of miles away.
Officials in Riyadh and the Saudi Embassy in Washington did not respond to requests for comment.
Having such a programme could further strain relations with the US, the kingdom's longtime security partner, at a time when ties already are being tested by the killing of Washington Post columnist Jamal Khashoggi and the Saudi-led war in Yemen.
Jeffrey Lewis, a missile expert at the Middlebury Institute of International Studies in Monterey, California, said heavy investment in missiles often correlates with an interest in nuclear weapons.
"I would be a little worried that we're under-estimating the Saudis' ambitions here," said Mr Lewis, who has studied the satellite images.
The images, first reported by the Washington Post, focus on a military base near the town of al-Dawadmi, some 145 miles (230km) west of Riyadh, the Saudi capital.
Jane's Defence Weekly first identified the base in 2013, suggesting its two launch pads appear oriented to target Israel and Iran with ballistic missiles the kingdom previously bought from China.
The November satellite images show what appear to be structures big enough to build and fuel ballistic missiles.
An apparent rocket-engine test stand can be seen in a corner of the base - the type on which a rocket is positioned on its side and test-fired in place. Such testing is key for countries attempting to manufacture working missiles, experts say.
Michael Elleman, senior fellow for missile defence at the International Institute for Strategic Studies in Washington, also reviewed the satellite photos and said they appear to show a ballistic missile programme.
The question remains about where Saudi Arabia gained the technical know-how to build such a facility.
Mr Lewis said the Saudi stand closely resembles a design used by China, though it is smaller.
Chinese military support to the kingdom would not come as a surprise. The Chinese have increasingly sold armed drones to Saudi Arabia and other Middle East nations, even as the US blocks sales of its own to allies over proliferation concerns. Beijing also sold Riyadh variants of its Dongfeng ballistic missiles, the only ones the kingdom was previously believed to have in its arsenal.
Asked by the Associated Press on Friday about the base, China's Defence Ministry declined immediately to comment.
"I have never heard of such a thing as China helping Saudi Arabia to build a missile base," Chinese Foreign Ministry spokeswoman Hua Chunying said.
Neither Saudi Arabia nor China are members of the Missile Technology Control Regime, a 30-year-old agreement aimed at limiting the proliferation of rockets capable of carrying weapons of mass destruction, such as nuclear bombs.
Saudi Arabia, along with Israel and the United States, have long criticised Iran's ballistic missile programme, viewing it as a regional threat.
Iran, whose nuclear programme for now remains limited by its 2015 deal with world powers, insists its atomic programme is peaceful. But Western powers have long feared it was pursuing nuclear weapons in the guise of a civilian programme, allegations denied by Tehran.
Iran has relied on its ballistic missiles as its own air force is largely made up of pre-1979 fighter jets. Saudi Arabia, on the other hand, has a fleet of modern F-15s, Typhoons and Tornadoes - which raises the question of why the Saudis would choose to develop the missiles.
Defence expert Mr Elleman said that, while Saudi pilots are skilled, the kingdom still needs American help with logistics.
"Today, they rely heavily on direct American support. There is no absolute guarantee that US forces and supporting functions will aid a Saudi attack on Iranian targets," he told AP. "Ballistic missiles are a reasonable hedge against those concerns."
Saudi Arabia, meanwhile, has been targeted by ballistic missiles fired from neighboring Yemen by the Houthi rebels, some of which have reached Riyadh. Researchers, Western nations and UN experts say Iran supplied those missiles to the rebels, something Tehran and the rebels deny.
Saudi Arabia is pursuing its own nuclear program,me and Prince Mohammed, the 33-year-old son of King Salman who is next in line for the throne, said it would race for an atomic weapon if Iran were to develop one.
"Saudi Arabia does not want to acquire any nuclear bomb, but without a doubt if Iran developed a nuclear bomb, we will follow suit as soon as possible," he told CBS's 60 Minutes in an interview aired last March.
A Saudi programme would only complicate efforts by the US and its Western allies to limit Iran's ballistic missile programme, said Stratfor, a private intelligence firm based in Austin, Texas.
Stratfor said "should Saudi Arabia move into a test-launch phase, the United States will be pressured to take action with sanctions", as it has done with Iran.
Congress has grown increasingly critical of Saudi Arabia since the October 2 assassination of Mr Khashoggi at the Saudi Consulate in Istanbul, allegedly carried out by members of Prince Mohammed's entourage. The kingdom's years-long war in Yemen also has angered politicians.
If the Saudis produce "medium-range systems inherently capable of carrying nuclear weapons, the response will be much more robust, though likely out of public view," Mr Elleman said.
"Congress, on the other hand, may lash out, as this will be seen as another affront to the US and regional stability."