The comments came after Trump unleashed a slew of fresh threats against North Korea on Friday, declaring the USA military "locked and loaded".
"Sim Tack, a senior analyst for private intelligence firm Stratfor, said the Japanese batteries are designed for protecting the area where they are deployed, "(they are) not meant to shoot missiles out of the sky as they pass over Japan at high altitude".
The possibility of a nuclear confrontation is considered remote but worldwide alarm has been escalating in recent days.
The now deployed radar of the USA missile defense system THAAD only monitors missiles originating from the north, and South Korea's Green Pine radar warning system are not sufficient for tracking submarine-launched attacks at sea, according to the report.
According to its reported plan, North Korea would fire four Hwasong-12 intermediate-range missiles over Japan and into waters around Guam, home to about 7,000 troops and 160,000 people.
Guam, an unincorporated USA territory, is a strategic outpost about 2,100 miles south-east of Pyongyang. At the northern end of the island, Andersen Air Force Base houses B-1 bombers as well as the Navy's Helicopter Combat Support Squadron Twenty-five. Their location in a USA territory means its military is just hours from potential flashpoints in the western Pacific.
The SDF spokesman said the country's Aegis ballistic missile defense system was deployed in the waters between Japan and the Korean Peninsula, but would not give a specific location.
"I'm pro military buildup", said resident Gus Aflague, 60, whose grandfather and brother both joined the U.S. Navy.
The exercises were scheduled well before tensions began to rise over Trump's increasingly fiery rhetoric and North Korea's announcement of the missile plan, which if carried out would be its most provocative launch yet. They threatened USA, other countries, and they threaten Guam.
"I feel it's different this time", said Yoon, who's been in the business since 2004. Guam stayed under naval rule until 1950, when the Organic Act made Guam a self-governing territory of the US and its residents became USA citizens.
The U.S. set up the Naval Station in Guam in 1899, designating the whole island as a base.
Andersen Air Force Base hosts a Navy helicopter squadron and Air Force bombers that rotate to Guam from the USA mainland. Guam was also a refueling and transfer spot for military personnel heading to Southeast Asia.
But there's also some resentment about not being able to vote in the general election for USA president. An air base with long-range bombers and a naval base that's home port for fast-attack submarines are among the most "strategically important" the US has in the Pacific, according to the CIA World Factbook.
"They believe that we are not true USA citizens, when we do have plenty of Chamorro people going into the USA military and serving for their country and dying for their country", he said.
Gofigan said there is not a lot of panic in Guam, and that people's faith - the island has been overwhelmingly Catholic since the arrival of Spanish missionaries in the 17th century- has been on display in recent days. "And not a lot of Americans know that".
On Friday, US President Donald Trump doubled down on his statement that he would unleash "fire and fury like the world has never seen" if Pyongyang continued its threats, saying in a tweet that "military solutions" were "locked and loaded" for use against North Korea.
Altogether, 7,000 US military personnel are stationed on Guam. Jennifer Sinco Kelleher in Honolulu and Jocelyn Gecker in San Francisco contributed reporting.