North Korea's missile program is evolving, experts say, as the country develops and produces missiles at a faster rate, and deploys them in ways which potentially help evade new and existing defenses.

"They did a launch a month ago, they're now launching (more) in 30 days, that's a third of the time they used to need," Carl Schuster, Hawaii Pacific University professor and former director of operations at the US Pacific Command's Joint Intelligence Center told CNN.

On Monday, North Korea launched four intermediate-range missiles into the Sea of Japan , also known as the East Sea, three of which landed within 200 nautical miles of Japan's coastline and the country's Japan's exclusive economic zone.

It came less than a month after North Korea launched the Pukguksong-2,a new type of missile which uses solid rocket fuel which allows for faster fueling before launch

Just over five months earlier, they fired three missiles more than 1,000 kilometers (620 miles) into Japan's air defense zone

"There's two ways to look at it -- one, they're quicker at doing the (launch) set up (and) they're quicker at making the missiles and transporting them," Schuster said.

"Keep in mind if they're doing a test shot you want to keep some (missiles) in reserve in case you go to war, so you're shooting a missile that is operationally spare. That means their missile stockpiles are larger than they were before."

Can North Korea overwhelm THAAD?

Not only is North Korea producing more missiles, their techniques are getting more advanced.

Jeffrey Lewis, adjunct professor at the Middlebury Institute of International Studies at Monterey, told CNN these launches were, in fact, North Korea's way of signaling that they're not testing missiles anymore.

"They're testing the units that fire the missiles. Can you fire them all at once to overwhelm a missile defense?" he said.

"This is the sort of behavior you see from a state that is planning to deploy nuclear weapons to its military units. That's a change that's important."

North Korea announced last September it had successfully detonated a nuclear warhead capable of being mounted on ballistic missiles, however some analysts doubt that claim.

Recent missile tests come as South Korea pushes to deploy a US-made THAAD missile defense system as a deterrent to any North Korean launches.

Lewis said multiple missiles fired simultaneously could be a technique to avoid THAAD, which stands for Terminal High Altitude Area Defense. "That's a basic missile defense countermeasure," he said.

"It's just harder for something like THAAD to deal with four targets at once rather than one. If you plan to use a nuclear missile on a target in South Korea, you want to fire them all at the same time."

Warning against South Korea drills

Most experts said North Korea's missile launch on Monday was in retaliation for the start of South Korea's annual military drills with the United States on March 1, also known as Foal Eagle.

"North Korea was looking for them to be scaled down somewhat so this maybe some way of expressing their discontent and warning the US and South Koreans that they are not going to tolerate being pushed around by large intimidating exercises," Josh Pollack, editor of the Nonproliferation Review told CNN.

Lewis said North Korea was demonstrating that any US or South Korean invasion would be met with nuclear force.

"The fact that they programmed against the US exercises is a message that now that they have nuclear weapons, on the first day of (a) war, they're not going to sit. They're going to use them," he said.

Launch likely to infuriate China

The launch comes as China, North Korea's closest international ally, convenes its annual National People's Congress in Beijing.

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"They won't be happy," Pollack said. "(North Korea) wants to signal they do not feel the need to provide formal warning to the Chinese and show deference to them anymore (although) they may be doing it in private."

It is just the latest example of Pyongyang antagonizing Beijing in the past six months.

In September, during China's big international moment at the G20 in Hangzhou, North Korea fired three missiles into the Sea of Japan

In February, North Korean agents were accused of murdering the exiled half-brother of Kim Jong Un, Kim Jong Nam, who was widely considered to be under China's protection . North Korea denies any involvement.

"They don't care (about China)," Lewis said. "There's always been a kind of relationship of coercion or bullying (between the two) ... It's fairly standard for North Korea to find that infuriating."