U.S. fears North Korean execution could be prelude to provocation WASHINGTON (Reuters) - North Korean leader Kim Jong Un's execution of his uncle last week is an example of the worrisome unpredictability of that regime and could be a prelude to some kind of provocation by Pyongyang, top Pentagon officials said on Thursday. The execution of Jang Song Thaek on Friday was the biggest upheaval in years in North Korea, which has conducted three nuclear tests and this year raised the possibility of nuclear war with South Korea and the United States. "These kind of internal actions by dictators are often a precursor to provocation to distract attention from what they're doing inside of that country," General Martin Dempsey, chairman of the U.S. military's Joint Chiefs of Staff, told a Pentagon news conference. But North Korean politics are virtually impenetrable from outside. Analysts have speculated, for instance, that Jang could have been purged over a falling out with Kim or other personal reasons. U.S. Defense Secretary Chuck Hagel said that uncertainty about North Korea's motives is "concerning to everyone." That nation is as closed as any nation in the world. There is no transparency," Hagel said. And so when you see things like this occur, it heightens the reaction of what people think ... could happen, with that kind of unpredictability. So, it's not a welcome event at all. Dempsey did not explicitly say what kind of provocation he feared from the North. The United States blames it for the 2010 sinking of a South Korean warship. Tensions were heightened earlier this year during U.S.-South Korean military drills that Pyongyang branded a "rehearsal for invasion." In a rare show of force during the drills last year, two nuclear-capable, bat-winged B-2 stealth bombers flew 37 1/2 hours from their U.S. base to drop dummy munitions on a South Korean range, and then returned home. The next edition of those annual drills are expected to be held in March, a Pentagon spokesman said. Reporting by Phil Stewart; Editing by Philip Barbara