Pyongyang is believed to be working toward building an atomic bomb small enough to mount on a long-range missile. Long-range rocket launches designed to send satellites into space in 2009 and 2012 were widely considered covert tests of missile technology, and North Korea has conducted three underground nuclear tests, most recently in February.
"I don't believe North Korea has the capacity to attack the United States with nuclear weapons mounted on missiles, and won't for many years. Its ability to target and strike South Korea is also very limited," nuclear scientist Siegfried Hecker, a senior fellow at the Center for International Security and Cooperation at Stanford University, said this week.
"And even if Pyongyang had the technical means, why would the regime want to launch a nuclear attack when it fully knows that any use of nuclear weapons would result in a devastating military response and would spell the end of the regime?" he said in answers posted to CISAC's website.
In Seoul, a senior government official said Tuesday it wasn't clear how advanced North Korea's nuclear weapons capabilities are. But he also noted fallout from any nuclear strike on Seoul or beyond would threaten Pyongyang as well, making a strike unlikely. He spoke on condition of anonymity because he wasn't authorized to speak publicly to the media.
North Korea maintains that it needs to build nuclear weapons to defend itself against the United States. On Monday, North Korean leader Kim Jong Un led a high-level meeting of party officials who declared building the economy and "nuclear armed forces" as the nation's two top priorities.
Hecker has estimated that North Korea has enough plutonium to make several crude nuclear bombs. Its announcement Tuesday that it would restart a plutonium reactor indicated that it intends to produce more nuclear weapons material.
The U.S.-Korea Institute at Johns Hopkins School of Advanced International Studies has analyzed recent commercial satellite imagery of the Nyongbyon nuclear facility, where the reactor was shut down in 2007 under the terms of a disarmament agreement. A cooling tower for the reactor was destroyed in 2008.
The analysis published Wednesday on the institute's website, 38 North, says that rebuilding the tower would take six months, but a March 27 photo shows building work may have started for an alternative cooling system that could take just weeks. Experts estimate it could take three months to a year to restart the plant.
Meanwhile, North Korea threatened to close the Kaesong industrial complex, which houses factories powered by South Korean money and know-how and North Korean labor. It is a symbol of inter-Korean cooperation that has endured years of declining relations.
Trucks carrying cargo and South Korean workers were turned back Wednesday, and again Thursday morning, at the border city of Paju.
North Korea was allowing South Korean managers at Kaesong to return home. About 220 South Koreans were to cross the heavily fortified border into the South throughout Thursday, according to Seoul's Unification Ministry.
Lee reported from Seoul. Associated Press writers Matthew Lee in Washington and Hyung-jin Kim and Youkyung Lee in Seoul contributed to this report. Follow AP's Korea bureau chief at www.twitter.com/newsjean.