PYONGYANG: A U.S. citizen has been arrested in North Korea, raising to three the number of Americans now detained by Kim Jong Un’s regime and adding to rising tensions between Washington and Pyongyang. South Korea’s Yonhap news agency reported that a former Korean-American professor was arrested at Pyongyang International Airport as he was attempting to leave the country. Yonhap described Kim as a former professor at the Yanbian University of Science and Technology in Yanji, Jilin, China. Yonhap said Kim, in his late 50s, had been involved in aid programs in North Korea. The reason for his arrest was not immediately available. The U.S. State Department said it was aware of reports that an American had been detained but declined to reveal details. “The protection of U.S. citizens is one of the Department’s highest priorities,” the State Department said in a statement. “In cases where U.S. citizens are reported to be detained in North Korea, we work with the Swedish Embassy, which serves as the United States’ Protecting Power in North Korea.” Park Chan-mo, the chancellor of the Pyongyang University of Science and Technology, identified the man to the Associated Press as Tony Kim. Park told AP Kim had come from the Chinese school and taught accounting in Pyongyang for about a month. Relations between the U.S. and North Korea have grown more strained in recent months, with Pyongyang continuing to conduct nuclear and ballistic missile testing in defiance of international bans. North Korea’s leader has been equally outraged by massive joint military exercises conducted by the U.S. and South Korea. Pyongyang said Sunday it was poised to destroy the USS Carl Vinson if provoked when the aircraft carrier and its armada arrive at the Korean Peninsula. The ship began exercises Sunday with Japan, the Navy said. Last week, Vice President Pence visited South Korea and Japan, warning that the U.S. response to any use of weapons by North Korea would be “overwhelming and effective.” “The United States of America will always seek peace but under President Trump, the shield stands guard and the sword stands ready,” Pence said.