(Bloomberg) -- China and Russia are deepening cooperation on a key atomic technology that has Pentagon planners on edge because of its potential to upset the global balance of nuclear weapons. 

Russian President Vladimir Putin and Chinese counterpart Xi Jinping announced a long-term deal to continue developing so-called fast-neutron reactors. The announcement came among a raft of agreements presented late on Tuesday following Xi’s three-day visit to Moscow, which also touched on topics from energy to news broadcasting.

In December, the Kremlin-controlled nuclear giant Rosatom Corp. finished transferring 25 tons of highly-enriched uranium to China’s first fast reactor, the CFR-600 — a facility which analysts say could produce fuel for some 50 nuclear warheads a year.

Congress has been pushing the White House to crack down on what it views as “dangerous” ties between Rosatom and the China National Nuclear Corp. 

“Russia and the People’s Republic of China’s nuclear cooperation goes much farther than just civilian projects,” wrote the chairmen of Congress’s armed services, foreign affairs and intelligence committees last week in a letter to National Security Advisor Jake Sullivan. 

Despite sanctions over its invasion of Ukraine, Russia’s nuclear exports have surged over the last year, boosting the Kremlin’s revenue and cementing its influence over a new generation of global buyers. It’s the world’s biggest supplier of nuclear reactors and fuel. China’s fast-reactors, which use liquid metal instead of water to moderate operation, are based on Russian technology.

US Department of Defense officials have repeatedly raised alarm over China’s nuclear-weapons ambitions since issuing a 2021 report to Congress. Military planners assess that the CFR-600 is poised to play a critical role in raising China’s stockpile of warheads to 1,500 by 2035 from an estimated 400 today. 

China has repeatedly rejected US concerns that the CFR-600 will be used to increase its nuclear-weapons stockpile. The unit is connected to the power grid and is part of the country’s ambitious $440 billion program to overtake the US as the world’s top nuclear-energy generator by the middle of next decade.

Speaking at a regular press briefing Thursday in Beijing, Foreign Ministry spokesman Wang Wenbin said that “China and Russia carry out normal civilian nuclear cooperation within the framework of international obligations and bilateral governmental agreements.”

“This is normal and beyond reproach,” he said.

(Updates with response from China’s Foreign Ministry.)

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