— -- What appears to be North Korea's largest nuclear test to date has raised concerns that the reclusive country is making progress in its nuclear weapons program, and quite possibly towards miniaturizing a nuclear warhead.
The United States and its partners in the region have expressed concern that North Korea might be able to place a miniaturized nuclear warhead atop a long-range missile, which could destabilize the region.
North Korea has conducted a slew of long-range and medium-range ballistic missile tests this year that show they are improving their missile capabilities.
ABC News looks at North Korea's advances in its nuclear and missile testing programs.
Why Another Nuclear Test Now?
Thursday's apparent nuclear test would be the second one this year by North Korea, which claimed that a test in January was a successful miniaturized hydrogen bomb test, though American officials discounted that claim. Based on initial seismic indications, it appears that Thursday's blast had a yield equivalent to 10 kilotons of TNT, North Korea's largest of the five nuclear tests it has conducted since 2006.
Two tests in one year reflects North Korea's experience in developing explosive devices quickly. It also indicates that North Korea wants to build on its stated goal of miniaturizing a nuclear warhead. Shortly after the January test, North Korean leader Kim Jong-Un posed next to a small device that North Korean media claimed was a miniaturized warhead, though that claim could not be independently verified.
A Pentagon spokesman said today that nothing had changed in the United States' assessment of North Korea's ability to miniaturize a nuclear warhead.
"We know what they’ve claimed that they’ve miniaturized," said Captain Jeff Davis, a Pentagon spokesman. "We’ve taken them at their word and have built a defensive architecture that assumes that capability. But we’ve never seen it demonstrated."
Thursday's test occurred on a national North Korean holiday that commemorated the 68th anniversary of the founding of North Korea.
How Advanced Has North Korea's Missile Program Become?
January's nuclear test was followed by a long-range ballistic missile test in February that successfully placed a North Korean satellite in orbit. U.S. officials claim that North Korea is using the cover of a satellite program to make advances on its long-range ballistic missiles.
That test was followed in the spring by the first ever launch tests of the Musudan mobile ballistic missile. The intermediate-range missile had drawn American concern for years because mobile missiles are harder to track by spy satellites and can be fired on short notice. But its capabilities had never been demonstrated. That changed this year when North Korea began testing the Musudan with often disastrous results. After five launch failures, the sixth Musudan missile finally succeeded and flew hundreds of miles into the Sea of Japan.
North Korea has also successfully launched a submarine-launched missile that also flew hundreds of miles into the Sea of Japan.
The variety of North Korean missile tests "suggests they are very intent on developing this capability across the board," Davis told Pentagon reporters today.
Each of these missiles could conceivably be armed with a miniaturized nuclear warhead if North Korea develops that capability. That would pose a threat to Japan and South Korea, which have already been outspoken in their concerns that North Korea could pose a physical threat to their countries.
What Can Stop North Korean Testing?
Kim Jong-Un appears to be unfazed by international criticism of North Korea's testing program and by international sanctions that ban testing. If anything, the pace of testing has accelerated since he became North Korea's leader. Three of North Korea's five nuclear tests have occurred on his watch. The accelerated level of missile testing this year indicates greater proficiency in developing ballistic missiles. North Korea is incorporating lessons learned from its launch failures as was indicated by the string of Musudan missile tests.
The united front presented by the United States and its regional allies does not have the weight that strong criticism from China would have on North Korea. Though China's Foreign Ministry has criticized the North Korean test and urged international dialogue, it remains unclear how recent tensions between the United States and China over the South China Sea could limit China's support for more international sanctions. In the wake of North Korea's missile tests this year, the U.S. has agreed to place a new anti-missile system in South Korea, but China is wary of the Terminal High Altitude Area Defense (THAAD) missile. It considers its placement on the Korean peninsula as a means of containing its own missile program.