"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.
Six US planes - including a "nuclear sniffer" and two Air Force B-1B Lancer bombers - took part in the military exercise on Tuesday night.
Separately, a small quake was detected early yesterday near the North's nuclear test site, South Korea's weather agency said, but unlike quakes associated with nuclear tests, it did not seem to be man-made.
"We have already warned several times that we will take counteractions for self-defense, including a salvo of missiles into waters near the U.S. territory of Guam", says Kim Kwang-Hak, a researcher at the Institute for American Studies at the North Korea Foreign Ministry. USA and South Korean warplanes also will join the exercise.
North Korea is preparing to fire multiple short-range rockets around the opening of the Chinese Communist Party's twice-a-decade congress on 18 October, a South Korean daily newspaper reported Wednesday.
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.
North Korea first said it was examining a plan to target the Pacific island in August after US President Donald Trump warned the isolated regime would "face fire and fury like the world has never seen" following a US intelligence assessment that North Korea had produced a miniaturized nuclear warhead.
Kim Jong-un's regime hit back today saying the war drill "hardened our determination that the U.S. should be tamed with fire".
Although they did not meet with Kim Jong-un, an interview with Ri-the former commanding general of the North Korean military-is considered the next-best opportunity.
Today's fresh threat was published in North Korean state media, according to CNN's Will Ripley who shared the statement on Twitter.
A Washington-based monitor of North Korea has also detected numerous landslides throughout the site, more than after any previous test.
'Let's hope that diplomacy works, ' he said.