By Rachel Lee

North Korea is struggling to develop technologies needed for its intercontinental ballistic missiles (ICBMs) to re-enter the atmosphere, according to the National Intelligence Service (NIS).

The NIS said the North has conducted several missile engine tests over the past weeks, but it has yet to perfect an ICBM.

In other words, it has yet to demonstrate a reliable re-entry vehicle robust enough to resist the heat and pressure of penetrating the atmosphere, the NIS explained.

The spy agency shared this information during a meeting with members of the National Assembly's Intelligence Committee, Thursday.

The re-entry technology seems a major hurdle for the North in its quest to develop an operational ICBM, the NIS said.

Re-entry technology refers to the ability to construct a shroud that covers the missile's warhead strong enough to withstand the extreme heat and other challenges involved in re-entering Earth's atmosphere from space.

Pyongyang has gone two months without missile tests since Sept. 15 when it fired a missile over Japan.

The NIS said the pause in missile tests could have come from a series of international sanctions and more pressure on North Korea has taken a heavy toll on its economy.

Financial difficulties from frequent missile tests under North Korea's leader Kim Jong-un's regime could be another factor that stopped provocations, the agency said.

North Korea conducted two tests for the "Hwasong-14" ICBM, which the North claimed it had perfected the warhead re-entry technology necessary to bring a nuclear-armed missile back into the Earth's atmosphere.

There is also speculation that the rotational deployment of strategic U.S. military assets around the Korean Peninsula, which is in progress, could have stopped Pyongyang from carrying out more provocations.

However, the intelligence agency said it is keeping a close eye on North Korea that could launch missile test anytime.

In September, the NIS forecast that the North could launch missile tests around key national events such as its regime foundation day on Sept. 9 or the anniversary of its ruling party's foundation on Oct. 10. Such predictions turned out to be wrong.

The U.S. government has not made any official comments about the pause in tests.

Jonathan Pollack, an analyst at Washington-based think tank, Brookings Institution, told USA Today that any answer is speculative.
"The reasons could be technical. The reasons could be political," Pollack said.

Pollack added that Kim may have "cooled the tests to see the reaction from President Donald Trump," who has repeatedly threatened to annihilate North Korea if it strikes first.

North Korea has traditionally fired fewer missiles in the last three months of the year, Tom Karako, an analyst at the Center for Strategic and International Studies, told the newspaper.

"We don't know why that is," Karako said.