John Bacon

USA TODAY

North Korea announced Sunday that it detained another American over the weekend, raising to four the number of U.S. citizens being held by the communist nation's authoritarian regime.

Kim Hak-song had worked at the Pyongyang University of Science and Technology, the same school where American Tony Kim had worked prior to being arrested at Pyongyang International Airport two weeks ago, North Korea's state-run KCNA news agency said.

Few details on Kim Hak-song or his arrest were immediately available. KCNA said he was detained Saturday on suspicion of committing “hostile acts" and that a "detailed investigation into his crimes" was underway.

The U.S. State Department issued a statement saying it was aware of the detention reports. The department works with the Swedish Embassy in Pyongyang, which serves as the United States’ Protecting Power in North Korea in dealing with such matters, the statement said. The U.S. has no direct diplomatic ties with Pyongyang.

"The security of U.S. citizens is one of the Department’s highest priorities," the statement said.

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North Korea confirmed Wednesday that it had detained Tony Kim, 55, who was seized April 22 as he tried to leave the country. Tony Kim, who taught accounting and other courses, is accused of attempting to overthrow the government.

"He was intercepted for committing criminal acts of hostility aimed to overturn the DPRK not only in the past but also during his last stay before interception," KCNA said in confirming reports of Tony Kim's arrest. DPRK is the acronym for the Democratic People's Republic of Korea.

Pyongyang University of Science and Technology is the only privately funded university in North Korea. It opened in 2010 and features several foreign teachers, unusual for North Korea. The school, which gets much of its funding from Western and South Korean charitable organizations, claims 500 undergraduate and 60 graduate students.

Other Americans being held in North Korea include Ohio native Otto Warmbier, 22, a University of Virginia student who was arrested in January 2016 while he was on a tour of North Korea, and Kim Dong Chul, 62, who was arrested in October 2015 while in North Korea on business.

Warmbier was accused of committing a "hostile act" for allegedly trying to steal a political banner and was sentenced in March 2016 to 15 years hard labor. Kim was accused of stealing military secrets and is serving a 10-year sentence.

Relations between the U.S. and North Korea have grown more strained in recent months as Pyongyang conducts nuclear and ballistic missile tests in defiance of international bans. North Korea leader Kim Jong Un has expressed outrage at massive joint military exercises conducted by the U.S. and South Korea and an anti-missile battery the U.S. is installing in the region.

On Friday, North Korea accused American and South Korean intelligence services of plotting to assassinate Kim Jong Un with "biochemical substances" in an effort to destabilize the isolated regime. The allegation followed a near unanimous vote in the U.S. House of Representatives on Thursday to toughen sanctions on North Korea and other countries that help its nuclear and missile programs, including China. The sanctions require Senate approval before being sent to President Trump to sign into law.