South Korea has gaps in its countermeasures against North Korean missiles — with or without America’s state-of-the-art defense system in place.

South Korea and the United States had to overcome political resistance from locals and diplomatic and economic pushback from China in order to get the anti-missile system known as THAAD deployed in South Korea. But the system has important limitations.

The first is its range. The Terminal High Altitude Area Defense system, which is currently positioned only in Seongju County of North Gyeongsang Province, is designed to intercept missiles within a range of 200 kilometers (124 miles). Southern parts of the country lie within that area, but the capital Seoul — by far the most densely populated area of the country — is not.

A second limitation is that THAAD can be overwhelmed. Even if it covered the Seoul metro area, it may not shoot down everything coming its way if the North were to fire multiple, short-range missiles — and Seoul is only about 30 kilometers (18.6 miles) from the border at its closest point.

“It’s harder to catch a low ball that comes in high speed than to catch a ball that comes at you in a parabolic trajectory. The same applies to a missile defense,” Professor Kim Dong-yub of the Institute for Far Eastern Studies told South Korea’s YTN news channel last week. “The THAAD becomes useless for South Korea if a missile comes below the interception altitude and at a high speed.”

South Korea’s Ministry of Foreign Affairs did not immediately respond to a CNBC request for comment.

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.

Trump tweet 5 September

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.”

But South Korea certainly doesn’t deny its need for better protection. Defense Minister Song Young-moo said in a press briefing this month that “South Korea will be considering options such as SM-3 for a multi-layer defense system, once the Aegis radar system is brought in.” Aegis refers to a complex, land-and-sea system pioneered by the U.S. Navy that uses elements from multiple defense contractors.

The problem is money. According to a calculation by South Korean media, it would cost the country about $1.7 billion for South Korea to load 20 SM-3 missiles onto three Aegis naval vessels, including the cost to renovate the ships. That would be about a fifth of the $10 billion allocated for the country’s military acquisition and research and development.

And there’s a competing item on the defense budget bill. The new president of South Korea promised during his campaign to increase monthly wages for soldiers. The military budget could get tighter for South Korea down the road if the president keeps that promise, according to Rand Corp.’s Bennett.

South Korea’s government decided last month to increase the defense budget for next year by 6.9 percent, to 43.1 trillion won, or $38.4 billion. The hike still needs the National Assembly’s green light. Even if the budget is passed, it’s unclear whether it would be sufficient to both boost military pay and expand the country’s missile defense.

—CNBC’s Huileng Tan contributed to this report