Missile experts suspect North Korea's most recent secretive test spells bad news for U.S. negotiations

North Korea is at it again. But this time no one is exactly sure about what they're up to.

North Korean state media reported Sunday that Pyongyang conducted a "successful test of a great significance" Saturday at its Sohae satellite launch site, a rocket testing ground, but did not reveal what was tested. U.S. officials have said North Korea promised to close the testing ground, but it appears that won't be the case any longer as Pyongyang's year-end deadline to reach a denuclearization agreement with Washington nears after talks stalled earlier this year.

It likely wasn't a missile launch, since Japan and South Korea can usually detect those. Instead, missile experts said it's possible North Korea tested a solid fuel rocket engine, which could allow the country to field intercontinental ballistic missiles that are easier to hide and faster to deploy. "If it is indeed a static engine test for a new solid or liquid fuel missile, it is yet another loud signal that the door for diplomacy is quickly slamming, if it isn't already," said Vipin Narang, a nuclear expert at the Massachusetts Institute of Technology. "This could be a very credible signal of what might await the world after the New Year."

North Korea has promised to adopt a "new path" if the U.S. does not offer sanctions relief, which analysts believe could include launching a satellite that would allow Pyongyang to continue testing missiles more covertly. Read more at BBC and Reuters.