North Korea is suspected of having built more nuclear warheads at a secret uranium enrichment facility in Kangson south of Pyongyang last year.

The Stockholm International Peace Research Institute estimates in its yearbook published Monday that North Korea had 30 to 40 nuclear warheads as of January this year, 10 more than last year's estimate.

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Shannon Kile of the institute told Radio Free Asia on Tuesday that it reached the conclusion from analysis of commercial satellite images, especially vehicular movements.

Facilities like the suspected one in Kangson, which the North has refused to declare, were the reason for the failure of the second U.S.-North Korea summit in Hanoi in February last year. North Korean leader Kim Jong-un only offered to dismantle the openly known nuclear facility in Yongbyon, which is thought to be more or less out of date.

Meanwhile, the U.S. Navy has deployed three aircraft carriers in the Pacific. The USS Ronald Reagan and Theodore Roosevelt are patrolling the western Pacific, which encompasses the Korean Peninsula and the South China Sea, and the USS Nimitz is sailing towards Asia after leaving San Diego, its home port.

Their simultaneous presence "represents the biggest deployment of U.S. aircraft carriers in the Pacific since 2017 -- when tensions with North Korea over Pyongyang's nuclear weapons program were at their peak," CNN said.

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