While there are concerns about the progress being made, analysts say that the reclusive nation does not yet have the ability to launch a strike on U.S. soil.

“They don’t have a proven ICBM capability and a warhead that could survive re-entry as I understand it,” Bonnie Glaser, senior adviser for Asia at the Center for Strategic and International Studies, told CNN by phone from Tokyo.

“So there are probably some technical difficulties there for hitting the United States.

“The point here is that with every test, the North Koreans are going to learn something and they’re going to make progress. And we probably should not underestimate their capability … if not today, then tomorrow.”

Escalation on the peninsula

The news comes as tensions are once again heightened on the Korean Peninsula, with the United States and South Korea conducting what has been described by a South Korean Defense Ministry spokesman as the “largest ever” joint military exercises, in which around 17,000 U.S. military personnel are participating alongside some 300,000 South Korean troops, according to U.S. Forces Korea.

North Korea on Sunday warned it would make a “pre-emptive and offensive nuclear strike” in response to the joint exercises.

The military spokesman said that the two allies were “closely monitoring” signs of North Korean provocation.

“As of now, there are no direct sign of provocation, but we are planning to continuously strengthen surveillance,” South Korean Defense Ministry spokesman Moon Sang-gyun said.

South Korea said it is keeping an eye on the situation with the help of U.S. intelligence authorities.

“South Korean Defense Ministry assesses North Korea, at this point, has not secured the capability of miniaturizing a nuclear warhead nor does it have actual combat capability of KN-08,” South Korea’s Defense Ministry said.

New nuclear threat from N. Korea over joint exercises 01:30

Analysts are questioning the wisdom of expansion of the annual exercises at a time when Pyongyang’s nuclear ambitions look as advanced as ever seen.

“I didn’t see the logic of expanding the exercises,” Stephan Haggard, professor at the School of Global Policy and Strategy, University of California, San Diego, and one of the authors of the North Korea: Witness to Transformation blog told CNN.

“I personally think that upping the sizes of the (joint U.S.-South Korea military) exercises didn’t serve any material function. It’s not clear that the size will bring North Korea back to the diplomatic table, so there’s no real purpose to do that.

“All you’ve done is stir the viper’s next. And the North Korean military and the leadership I’m sure is extremely nervous. Because it’s coming in the context of the sanctions, and the Chinese are clearly displeased.”

The North Korean warning also follows last week’s sanctions announced by the 15-member U.N. Security Council, which Pyongyang has denounced as “unprecedented and gangster-like.”

Bomb test

Discussions about new sanctions started after North Korea claimed to have successfully tested a hydrogen bomb in January, its fourth nuclear test.

Then, in February, Pyongyang said it had successfully launched an Earth satellite into orbit via the long-range Kwangmyongsong carrier rocket.

How close is North Korea to weaponizing a rocket?

The Security Council called those moves “violations and flagrant disregard” of previous resolutions.

KCNA claims that Kim has visited a facility where warheads had been made to fit on ballistic missiles. CNN cannot independently verify the images accompanying the story.

On Friday, KCNA reported that Kim said his country’s “nuclear warheads need to be ready for use at any time.”

“Under the extreme situation that the U.S. imperialist is misusing its military influence and is pressuring other countries and people to start war and catastrophe, the only way for our people to protect sovereignty and rights to live is to strengthen the quality and quantity of nuclear power and realize the balance of power,” Kim said, according to KCNA.

This rhetoric came out a day after the news agency reported tests of a new multiple-launch rocket system. This may or may not be referring to a launch of “short-range projectiles” chronicled one day earlier by the South Korean Joint Chiefs of Staff.

Pyongyang has long boasted about its nuclear ambitions, about as long as South Korea and the United States have sought to derail them. The issue has only furthered the isolation of North Korea, a communist, closed-off state led for decades by the authoritarian Kim, his late father and his grandfather.

A chief concern is not only that Pyongyang will develop effective nuclear warheads, but that they’ll pair them with missiles that can strike targets around East Asia and perhaps beyond.