* North Korea "hardest target" for intel, U.S. says
* Motivations unclear for sinking of South Korean warship
* Competing theories on Cheonan sinking include succession
By Phil Stewart
WASHINGTON, Sept 16 (Reuters) - The United States said on Thursday it lacked intelligence on North Korea and could only guess whether the reclusive state's leader, Kim Jong-il, will be succeeded by his youngest son.
The blunt acknowledgment before a Senate panel by the top State Department official responsible for the region underscores the uncertainty complicating U.S. and South Korea policy toward a regime they say threatens regional stability.
"In fundamental ways, North Korea is still a black box. We have some glimpses and some intelligence and the like, but the truth is often times in retrospect some of that intelligence has proven to be wrong," said Kurt Campbell, Assistant Secretary of State, East Asian and Pacific Affairs.
"It's a very, very hard target, probably the hardest target we face in the global arena."
U.S. Defense Secretary Robert Gates -- acknowledging some uncertainty -- has linked the March sinking of a South Korean warship, which Washington and Soul blame on Pyongyang, to the apparent succession of Kim Jong-il.
Gates last month said he had a "sneaking suspicion" Kim Jong-il's youngest son was seeking to take over in North Korea and sought to earn his stripes with the North's military through the Cheonan's sinking.
Gates and other U.S. officials have raised concerns about future "provocations" until succession for Kim, 68, is resolved.
Campbell refused to speculate about the succession in the Senate hearing.
When asked by Senator John McCain whether the U.S. expected Kim Jong-un to replace his father, Campbell quipped: "Your guess is as good as ours, Senator."
"Well that's an interesting comment on our intelligence capability in North Korea," replied McCain, who lost to Barack Obama in the 2008 presidential election. Campbell later apologized for what he called a "flip" response and offered a more extensive appraisal of intelligence shortfalls.
Wallace Gregson, Assistant Secretary of Defense, Asian and Pacific Security Affairs, said there were two main theories for North Korea's motivations for the March sinking of the Cheonan, which killed 46 South Korean sailors.
One was related to "mysterious" succession politics inside North Korea. The other was that it was a reaction to a November 2009 naval firefight between the North and South Korean naval forces.
"I'm not personally aware that anybody has a definitive answer beyond those two competing theories," Gregson said.
Campbell then jumped in: "Can I just add a third to that, if I could." He said the sinking also might be linked the upcoming G20 Summit scheduled for Seoul in November, since past provocations came ahead of important events for South Korea.
"The upcoming G20 is a very big deal, very big deal, for the South Koreans. It's the arrival of South Korea on the global stage -- probably again the biggest deal in their history," he said.
"But I have to underscore that in many respects, this is a black box. We really don't know about the motivations, ultimate goals and ambitions behind such a dangerous and provocative act." (Additional reporting by David Alexander; Editing by Jerry Norton)
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