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Lyle Morris, a senior fellow at the Asia Society Policy Institute, told the BBC that developments such as hypersonic missiles were making China reconsider its second-strike policy, forcing the expansion of its stockpile.

“He is determined to operationalize his nuclear arsenal and is developing a credible second-strike capability.”

The weapons information “made an important contribution to the Chinese objective to maintain a second-strike capability and provided useful information for future designs,” the assessment said.

“Japan wanted to limit its defense spending and try not to acquire second-strike capability. But the situation surrounding us does not permit us to do that,” said Ichiro Fujisaki, former Japanese ambassador to the United States.

The Arctic is home to Russia's Northern Fleet, its key fighting force in the region, tasked with Russia's nuclear second-strike capacity - its ability to respond to a nuclear attack using its own nuclear weapons against the attacker.