The U.S. Defense Department said it was deploying a missile defense shield to the Pacific as North Korea renewed its threat of a nuclear attack on the United States using "smaller, lighter and diversified" weapons.
The almost-daily aggressive moves by Pyongyang are seen as a test of the reclusive country's young new leader, Kim Jong Un, and concerns over whether he'll follow talk with action have rattled the region.
Defense Secretary Chuck Hagel called North Korea's rhetoric a "real and clear danger" and a threat to the U.S. and its Asia-Pacific allies. He said the U.S. is doing all it can to defuse the situation, which has included threats against the U.S. military base in Guam in the Pacific, Hawaii and the U.S. West Coast.
The Pentagon said it will deploy a high-altitude missile defense system to Guam, a U.S. territory, to strengthen the Asia-Pacific region's protections against a possible attack. The Pentagon already has sent dispatched bombers, stealth fighters and ships to the region.
Tensions have escalated in recent weeks as the U.S. and South Korea hold regular joint military drills. North Korea also is angry with a new round of U.N. sanctions over the country's latest nuclear test.
The communist North has vowed to increase production of nuclear weapons materials.
It has already begun construction at a closed plutonium reactor that it vows to restart, and it could be back in operation sooner than expected, a U.S. research institute said.
The North has already conducted three underground nuclear tests. Plans to restart the reactor and increase production of atomic material underscore worries about its progress in developing a nuclear-tipped missile that could target the United States. It is still believed to be years away from achieving that.
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.
"Pyongyang may be poised to prove wrong conventional wisdom that it will take months to restart its reactor, and in the bargain it is also showing us that they mean business by accelerating the process of producing more material for nuclear weapons," said Joel Wit, 38 North editor and a former U.S. State Department official.
North Korea also said it would restart a uranium enrichment plant. Both facilities could produce fuel for nuclear weapons.
State Department spokeswoman Victoria Nuland told reporters Tuesday that restarting the plutonium reactor would be "extremely alarming" but added: "There's a long way to go between a stated intention and actually being able to pull it off."
The new construction at the reactor began in the six weeks between February 7 and March 27, when another aerial image showed no building going on, 38 North says.
The analysis says the construction could be aimed at connecting the reactor to a pumping station that serves an adjacent light-water reactor that is still under construction. The light-water reactor still appears more than a year away from becoming operational, but its pumping station appears from aerial imagery to be complete, it says.
Restarting the plutonium reactor would also depend on the availability of fresh fuel rods. According to 38 North, North Korea is believed to have a supply of rods, but many may need adapting for use in the reactor.
The North's plutonium reactor began operations in 1986 but was shut down as part of international nuclear disarmament talks in 2007 that have since stalled. Once it is up and running, the reactor is capable of producing 6 kilograms (13 pounds) of plutonium a year — enough for one or two bombs, the analysis says.