North Korea’s military is clandestinely building a nuclear-powered submarine, according to a Japanese newspaper report, the latest provocation by Pyongyang in an escalating clash with the U.S. and its allies in a region already on edge.

The report by Japan’s Sekai Nippo, citing an “informed” but unidentified “source familiar with the North Korean situation,” said the size of the nuclear-powered submarine under construction is unclear but that the Kim Jong-un regime in Pyongyang hopes to have it deployed within three years.

The claim could not be independently verified by The Washington Times, and U.S. intelligence sources could not immediately be reached for comment. If true, however, the claim could indicate a dramatic step forward for North Korea’s navy, which analysts estimate operates a fleet of 50 to 60 diesel-electric submarines that are louder and easier to detect than the nuclear-powered vessels.

North Korea is expected to be a prime focus of President Trump’s week in New York meeting with other world leaders at the U.N. General Assembly. On Twitter, Mr. Trump revealed that he had spoken with South Korean President Moon Jae-in Sunday about Pyongyang. His tweet mocked Mr. Kim as “Rocket Man” and boasted about the effectiveness of international sanctions to cut off the North’s energy supplies.

“President Trump and President Moon committed to continuing to take steps to strengthen deterrence and defense capabilities, and to maximize economic and diplomatic pressure on North Korea,” the White House said in a statement. Mr. Kim, who rarely travels, will not be in New York, but North Korean Foreign Minister Ri Yong-ho is expected to address the world body Friday.

The Sekai Nippo report suggested a nuclear-powered submarine would dramatically increase the threat posed to the U.S. and its allies by North Korean submarine-launched ballistic missiles.

Pyongyang has quietly carried out as many as six test launches of the Pukguksong-1 SLBM since 2014, according to 38 North, a website that analyzes North Korea at John’s Hopkins University’s School of Advanced International Studies in Washington.

The submarine could be used to move the Pukguksong-1 SLBM into international waters in a much quieter and harder-to-detect manner capable of evading existing U.S., Japanese and South Korean missile defenses.

Pyongyang has dramatically accelerated its nuclear weapons and land-based ballistic missile programs, raising the prospect that it will soon have a nuclear device it could deliver to much of the U.S. mainland. A small nuclear device also could be put on the sub.

But analysts say the status of North Korea’s SLBM program is murky.

U.S. officials have increased their scrutiny of Pyongyang’s naval operations following reports last month that the North Korean navy may have carried out an “ejection test” relating to the SLBM program.

In May, 38 North published an analysis claiming that commercial satellite imagery had identified a second submersible test stand barge for North Korea’s SLBM program at Nampo Naval Shipyard, located on the nation’s western coast.

The barge appeared to be identical in size and layout to an original barge first seen in 2014 at the Sinpo South Shipyard on the nation’s eastern coast and resemble the Russian PSD-4 submersible missile-test barge.

The Sekai Nippo report said Chinese and Russian engineers are involved in North Korea’s push to build a nuclear-powered submarine and that the construction has been occurring clandestinely inside a dock near Nampo since January.

Nampo is a port city roughly 30 miles from Pyongyang and is home to a concentration of factories manufacturing electronic and machine products, making it “ideally suited for building a submarine,” the Japanese newspaper said.

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