In addition to THAAD, South Korea has around eight batteries of the Patriot defense system, and the United States separately operates roughly the same number there. But even the combined efforts of the systems isn't enough to counter an overwhelming missile attack, some experts say.
"Patriot has a much smaller range. North Korea can fire outside those areas," Rand Corp. defense analyst Bruce Bennett told CNBC, adding that "these batteries are not mutually reinforcing. They tend to be spread out to different places." The exact locations of those Patriots are classified.
Against the backdrop of recent missile and nuclear tests by North Korea, reports out of South Korea have suggested the possibility of the country buying an upgraded version of Patriot or an interceptor system called the SM-3. Japan already uses those batteries.
THAAD is built by Lockheed Martin, while both the Patriot and SM-3 are manufactured by Raytheon.
Following North Korea's nuclear test earlier this month, U.S. President Donald Trump declared on Twitter that he would let South Korea and Japan expand their military purchases from the United States.
The White House formally announced that the president "gave his conceptual approval for the purchase of many billions of dollars' worth of military weapons and equipment from the United States by South Korea" in a phone call with South Korea President Moon Jae-in. South Korea's presidential office said only that they discussed ways "to adopt the United States' state-of-the-art weapons and technology in response to evolving military threats from the North."