Story highlights North Korea's recent missile tests may be aimed at THAAD, experts say

The missile defense system could be bypassed by high density missile attacks or longer range missiles

Hong Kong (CNN) It's been a busy week for arms tests in North Korea. They fired three ballistic missiles on Monday, followed by the test explosion of a nuclear warhead on Friday.

The pace of North Korea's weapons program seems to be increasing, raising speculation that it may be trying to upgrade its arms before the deployment of a controversial US missile defense system in South Korea.

The simplest way to bypass a missile defense system is to just launch more missiles than it can effectively intercept, says Jeffrey Lewis, director of the US-based East Asia Nonproliferation Program

"Launching them simultaneously is more difficult for a missile defense system," he says. .

If those missiles are equipped with nuclear warheads "they don't have to get too many up in the air and past the missile defense system to have an effect."