The USS Michigan nuclear-powered submarine enters the naval base of Busan, South Korea, on October 13, 2017.
"We have already warned several times that we will take counteractions for self-defence, including a salvo of missiles into waters near the USA territory of Guam", Mr Kim, the researcher, told the North's official Korean Central News Agency.
The 10-day drill, which involves the aircraft carrier Ronald Reagan, is set to begin on Monday in waters east and west of South Korea.
The U.S. military said in a separate statement it conducted drills with Japanese fighters after the exercise with South Korea, making it the first time U.S. bombers have conducted training with fighters from both Japan and South Korea at night.
A nuclear-powered attack submarine also made a high-profile visit to a South Korean port.
In August, North Korea threatened to fire a salvo of intermediate range missiles toward the U.S. Pacific island territory.
After US B-1B jets buzzed North Korea and practiced a bombing run, Pyongyang responded by saying Kim Jong Un may finally decide to launch missiles towards Guam.
"We have already warned several times that we will take counteractions for self-defense, including a salvo of missiles into waters near the USA territory of Guam", Kim Kwang Hak, the researcher, told the North's official Korean Central News Agency on Friday.
Today's fresh threat was published in North Korean state media, according to CNN's Will Ripley who shared the statement on Twitter. But Trump has added to tensions in recent weeks by employing similarly tough talk, threatening to "totally destroy" or rain down "fire and fury" on North Korea. Only 16 percent of Democrats favor the idea of the US making a preemptive strike on North Korea, while a whopping 77 percent of Democrats oppose it.
Kelly added that Americans should be concerned that the North is getting closer to achieving the ability to hit the mainland USA with its missiles.
But he stipulated there is "great concern" for the citizens of Guam, which features a significant number of USA military installations.
"Right now we think the threat is manageable", Kelly told reporters, via a live feed. Over 60 percent said they do have confidence in "top national security and diplomatic officials" to handle the situation.
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