North Korea confirmed Wednesday that it detained a U.S. citizen in April for committing what it described as "criminal acts of hostility" against the country.

The state-run Korean Central News Agency identified the citizen as Kim Sang Dok, who was invited to Pyongyang University of Science and Technology to teach accounting as a professor.

North Korea believes Kim, detained on April 22 at Pyongyang's international airport, was trying to "overturn" the country's system through his acts in the past and during the latest stay, KCNA said, without providing further details.

The detention was announced amid growing worries over North Korea's nuclear program and high tensions between it and the United States.

On April 23, the university released a statement saying that the Korean-American, also known as Tony Kim, was detained at the airport as he was to leave North Korea.

The statement said he had been teaching for several weeks at the university, but it could not comment on whatever he allegedly did as it was unrelated to his teaching work on the campus.

His detention has brought the total of U.S. citizens being held in North Korea to three. The two others are Kim Dong Chul, a Korean-American man, and Otto Warmbier, a university student.

About one year ago, North Korea sentenced the student, now 22, to 15 years of hard labor for attempting to steal an item with a political slogan from a staff-only hotel area and the 63-year-old Korean-American man to 10 years of hard labor for subversive activities.

North Korea has a track record of detaining foreigners to use as bargaining chips in disputes with the United States and other countries.

U.S. President Donald Trump's administration has warned North Korea that all options, including a military strike, are on the table and sought to work with China to put more pressure on Pyongyang.

Amid signs of preparing another nuclear test or a ballistic missile launch, the U.S. administration has sent an aircraft carrier-led strike group to waters off the Korean Peninsula.

North Korea has repeatedly argued that its pursuit of nuclear weapons is aimed at deterring military threats posed by the United States, with which it has never had diplomatic relations.

At the same time, North Korea has in the past signaled its interest in engaging in negotiations with the United States for the purpose of drafting a permanent peace treaty to replace the armistice that stopped, but technically never ended, the 1950-1953 Korean War.