North Korea detained a United States citizen on Sunday amid rising tensions between the two countries. However, the reasons for his detention have not been released.

Pyongyang University of Science and Technology Chancellor Park Chan-mo, who identified the detained American as Tony Kim, said officials held the 50-year-old professor in Pyongyang when he was trying to flee the country. Kim is also known by his Korean name Kim Sang-duk. He had taught accounting at the university for about a month. Prior to that, he used to teach at the Yanbian University of Science and Technology in China, reported The Guardian.

The Swedish embassy, which monitors consular affairs for US in North Korea because the two countries do not have diplomatic relations, also confirmed the detention. However, the detention was “not connected in any way” to the university, reported BBC.

The detention comes as tensions between North Korea and the US have been escalating. With this, three US citizens are currently being held by North Korea. In April last year, Kim Dong-chul was sentenced to 10 years’ of hard labour for espionage. He was arrested in October 2015. In January 2016, a 21-year-old student, Otto Warmbier, as arrested for trying to steal a propaganda banner. He was also sentenced to 15 years’ of hard labour. The US has accused North Korea of detaining its citizens to use them as pawns, according to BBC.

Meanwhile, North Korea on Sunday threatened to sink a US aircraft carrier as a show of its military powers. The threat came after two Japanese navy ships joined the group for exercises in the western Pacific. “Our revolutionary forces are combat-ready to sink a US nuclear-powered aircraft carrier with a single strike,” the Rodong Sinmun, the newspaper of the ruling Workers’ Party, said in a commentary.

Tension started brewing on April 7 when US President Donald Trump had ordered a missile attack in Syria as retaliatory measure to a deadly chemical attack. On April 11, the North Korean state media warned of a nuclear attack on the United States at any sign of American aggression. Soon after, the US dropped its largest non-nuclear bomb on Afghanistan. The US Navy had also deployed a carrier group in the region and expressed concerns over North Korea’s nuclear power. North Korea, on the other hand, has defended its nuclear powers as self defence.

Last week, China had urged both the US and North Korea to refrain from mounting pressure on each other before it reaches an “irreversible and unmanageable stage”. The country’s Foreign Minister Wang Yi said, “We call on all parties to refrain from provoking and threatening each other, whether in words or actions.” Wang further hinted that a conflict could break out at any moment, adding that there will be no winner if such a situation arises. “If a war occurs, the result is a situation in which everybody loses and there can be no winner,” he had told reporters at Beijing.