Despite China’s ramped-up military presence and strategy in the South China Sea, U.S. naval officials still believe North Korea is the biggest threat to the United States in the Pacific region. Admiral Harry B. Harris Jr., who assumes leadership of the U.S. Pacific Command at Pearl Harbor in Hawaii Wednesday, said that North Korea’s volatility is of grave concern to American allies in the region.

“The greatest threat we face is North Korea,” Harris said in an interview with Time magazine. “They have an unpredictable leader who is poised, in my view, to attack our allies in South Korea and Japan.”

In the past, North Korean leadership, led by Kim Jong Un, has made threats of cross-continental missile strikes. Though threats have not came to fruition, in 2013 North Korean newspapers revealed that Kim had held military meetings where “U.S. mainland strike” plans were discussed. “He is on a quest for nuclear weapons and the means to deliver them intercontinentally,” Harris said. While it is believed Pyongyang doesn’t yet have the technology to perform strikes of that distance, local governments in Hawaii, southern California and Texas have all treated the mounting tensions as a serious threat.

Harris also points to Kim’s erratic leadership as a key concern for the region. “He kills people around him who disagree with him, and that’s something we should always keep in mind.”

Harris is likely referring to the recent slew of executions and demotions of high-ranking military officials that were ordered by Kim. Most recently, Hyon Yong Chol, North Korea’s defense minister, was reportedly executed by Kim for reportedly falling asleep during a high-level meeting. Following those reports, South Korean newspapers reported continued purging of military officials. Kim also made headlines in 2013, when he executed his uncle by marriage, Jang Song Thaek, a longtime aide of Kim’s father and predecessor Kim Jong Il, and a key figure in developing economic ties with regional ally China.

According to South Korean newspaper Chosun Ilbo, since Kim took the helm in late 2011, the average tenure of top officials in North Korea does not typically exceed one year.

As Harris embarks on his new position, which covers America’s extensive military naval operations with allies and enemies from California to the Indian Ocean, North Korea still manages to be a priority when it comes to U.S. security strategy in the region.

“North Korea keeps me up at night.”