At least two THAAD batteries are necessary for South Korea to better defend against North Korea's submarine launched ballistic missiles as a single unit can't cover all of the North's seas, a US arms expert said Monday.

Increasing THAAD batteries, however, doesn't guarantee full defense against the North's SLBMs because it's questionable whether THAAD is capable of intercepting missiles that are launched at high angles and involve atmospheric reentry, said Jeffrey Lewis, director of the East Asia Nonproliferation Program at the James Martin Center for Nonproliferation Studies.

"THAAD has a forward-looking radar with a 120-degree field of view. In the case of a single THAAD battery, North Korea's submarines would not have to travel very far out to sea to attack the THAAD system from behind the field of view of its radar," Lewis said, referring to the North's SLBM.

"South Korea needs two THAAD batteries to better cover ocean approaches. That is an obvious solution to at least part of the problem posed by the KN-11," the expert said in an article posted at Arms Control Wonk, a blog website he founded.

The assessment came after the North successfully conducted the latest SLBM test last week, sending the missile, designated KN-11, some 500 kilometers over the East Sea, the greatest distance the communist nation has achieved since it began SLBM tests last year.

Moreover, the missile was launched at a high angle, meaning the missile could have flown farther.

"North Korea fired it nearly straight up, reducing the range.

If fired on a minimum trajectory, the KN-11 would have traveled much further than 500 km over 1,000 km and probably much further," Lewis said.

Deploying two THAAD units, however, "does little to address the possibility of lofted attacks," he said.

"Lofting a long-range missile results in reentry at very high speeds and at a very severe angle. Whether THAAD can deal with a lofted KN-11 depends in part on the missile's range," he said.

"There was a healthy debate in 1990s about THAAD's ability to intercept warheads reentering at higher speeds."

THAAD is designed to intercept medium-range ballistic missiles with ranges of 1,000-3,000 kilometers and in theory should have some capability to intercept intermediate-range missiles with ranges of 3,000-5,500 km. But the system has never been tested against an intermediate-range target, let alone on such an unusual angle of attack, Lewis said.

The Pentagon's testing office -- the office of Director, Operational Test and Evaluation (DOT&E) -- gives THAAD a good rating against medium-range ballistic missiles, but the lowest possible rating against Intermediate-range ballistic missiles, he said.

The expert stressed, however, that beefing up defenses could lead only to accelerating arms races.

"South Korea has every right to defend itself against ballistic missile attack, both through THAAD and indigenous missile defenses.

I don't understand the public opposition to THAAD in South Korea," he said. "But even so, I don't think we can arms-race our way out of vulnerability with North Korea."

He said the North has too many countermeasures to defeat missile defenses.

"Deploying defenses and precision-strike capabilities will intensify the arms race rather than provide an escape from it," he said, adding that arms-racing is like a Gordian knot, "where our best efforts to wriggle free of vulnerability only tighten the ropes."

"Our best option, unpalatable as it may be, involves finding ways to discourage North Korea from developing new capabilities.

Defense is a far less effective strategy," he said. (Yonhap)