A third US citizen has been arrested and remains in custody in North Korea, according to South Korean news agency Yonhap. A man, a Korean-American professor in his 50s, identified by the surname Kim, had been in North Korea for a month to discuss relief activities and was detained at Pyongyang International Airport just as he was leaving North Korea, the agency reported.

The man was a former professor at Yanbian University of Science and Technology (YUST), Yonhap said, citing unnamed sources. YUST, a university in neighboring China, has a sister university in Pyongyang. An official at South Korea's National Intelligence Service said it was not aware of the reported arrest. The reason for his arrest is still unclear, and there has been no comment from the US authorities so far. South Korea’s spy agency, the National Intelligence Service, said it “was not aware” of Kim’s arrest, according to Yonhap.

North Korea, which has been criticized for its human rights record, has in the past used detained Americans to extract high-profile visits from the United States, with which it has no formal diplomatic relations. North Korea already holds two Americans. Ahn Chan-il, director of the World North Korea Research Center in Seoul, said that the North "seems to be intending to use professor Kim as leverage in negotiations" amid the current bad relations between the two countries.

Pyongyang airport

According to Reuters, Otto Warmbier, a 22-year-old student, was detained in January last year and sentenced to 15 years of hard labor by a North Korean court for attempting to steal a propaganda banner. In March 2016, Korean-American Kim Dong Chul, 62, was sentenced to 10 years hard labor for subversion. U.S. missionary Kenneth Bae was arrested in 2012 and sentenced to 15 years hard labor for crimes against the state. He was released two years later.

Since 2009, over 10 U.S. citizens have been detained in North Korea on charges of anti-state and other unspecified crimes. The widespread view has been that Pyongyang uses the detentions as bargaining chips in its negotiations with Washington.

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Meanwhile, one day after North Korea lashed out at its biggest supporter in the region China, threatening Beijing with "catastrophic consequences" for siding with the U.S. over sanctions, North Korea said on Sunday it was ready to sink a U.S. aircraft carrier steaming toward the Korean penninsula to demonstrate its military might, as two Japanese navy ships joined a U.S. carrier group for exercises in the western Pacific.

"Our revolutionary forces are combat-ready to sink a U.S. nuclear-powered aircraft carrier with a single strike," the Rodong Sinmun, the newspaper of the North's ruling Workers' Party, said in a commentary. The paper compared the aircraft carrier to a "gross animal" and said a strike on it would be "an actual example to show our military's force". The commentary was carried on page three of the newspaper, after a two-page feature about leader Kim Jong Un inspecting a pig farm.

North Korea says its nuclear program is for self-defense and has warned the United States of a nuclear attack in response to any aggression. It has also threatened to lay waste to South Korea and Japan.

Additionally, as discussed previously, North Korea will mark the 85th anniversary of the foundation of its Korean People's Army on Tuesday when some speculate it may set off a nuclear test.

Donald Trump ordered the USS Carl Vinson carrier strike group to sail to waters off the Korean peninsula in response to rising tension over the North's nuclear and missile tests, and its threats to attack the United States and its Asian allies. The United States has not specified where the carrier strike group is as it approaches the area. U.S. Vice President Mike Pence said on Saturday it would arrive "within days" but gave no other details.

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Adding to the growing military tension in the region, two Japanese warships, the Samidare and Ashigara, left western Japan on Friday to join the Carl Vinson and will "practice a variety of tactics" with the U.S. strike group, the Japan Maritime Self Defense Force said in a statement according to Reuters.

The Japanese force did not specify where the exercises were taking place, but by Sunday the destroyers could have reached an area 2,500 km (1,500 miles) south of Japan, which would be east of the Philippines. From there, it could take three days to reach waters off the Korean peninsula. Japan's ships would accompany the Carl Vinson north at least into the East China Sea, a source with knowledge of the plan said. U.S. and South Korean officials have been saying for weeks that the North could soon stage another nuclear test, something the United States, China and others have warned against.

Japan's show of naval force reflects growing concern that North Korea could strike it with nuclear or chemical warheads. Some Japanese ruling party lawmakers are urging Prime Minister Shinzo Abe to acquire strike weapons that could hit North Korean missile forces before any imminent attack.

Japan's navy, which is mostly a destroyer fleet, is the second largest in Asia after China's.

South Korea has already put its forces on heightened alert. China, North Korea's sole major ally, opposes Pyongyang's weapons programs and has appealed for calm. The United States has called on China to do more to help defuse the tension. Last Thursday, Trump praised Chinese efforts to rein in "the menace of North Korea", after North Korean state media warned the United States of a "super-mighty pre-emptive strike".