But as Pyongyang ramped up its war rhetoric, the United States flew stealth bombers from the state of Missouri to South Korea and moved a missile defense system to the island Guam. South Korea dispatched war ships to patrol its waters. Korean intelligence announced it was tracking North Korean missile launchers moving to the east, but no large movements of troops or hardware.
Meanwhile, North Korea's only ally, China, has made no visible military posturing of its own, despite a 1961 mutual defense treaty.
China has long valued stability in the Korean peninsula to prevent large refugee flows across the border. But, analysts say those concerns are exaggerated.
Carl Baker, director of programs at the Pacific Forum Center for Strategic and International Studies in Honolulu, said there are no verifiable reports of any significant military buildup along the border.
“China remains concerned about a flow of refugees but it's not it's not like it's going to be a big problem for China," said Baker. "If you look at the numbers, you know, the number of refugees that could move out of North Korea would be an inconvenience but it certainly wouldn't be a disaster for China to be able to handle folks that came over the border. But, again, I don't see any real evidence that there's a large number of troops moving up in that region.”
Earlier this month, western media outlets, quoting unnamed sources, reported that there have been signs of a Chinese troop buildup on the border with North Korea in in preparation for refugees should war break out. The German news agency DPA reported China's buildup was also aimed at securing North Korea's nuclear facilities, while a Washington Times report said it indicated Beijing was willing to back Pyongyang militarily. The unconfirmed reports were translated and repeated on some Chinese websites and blogs but dismissed by most analysts.
In public comments China's Ministry of Defense has only repeated statements urging all sides to ease tensions, initiate dialogue and maintain regional peace and stability.
China shares a 1,400 kilometer border with North Korea, five times longer than the border with South Korea, known as the demilitarized zone.
Unlike the heavily guarded border between the North and the South, the border with China, though watched by the military, is surprisingly open. Beijing in recent years built fences along parts of the border but much of it remains porous, including narrow areas of the Tumen River that freeze in winter and can be easily walked across.
It is still in Beijing's interest to prevent conflict that could bring U.S. troops to its borders, said Euan Graham, senior fellow at Nanyang Technological University in Singapore.
"The logic of North Korea as a buffer still, I think, has currency within, particularly military circles in China," he said. "I think there is an element that in a broader, strategic view, China is calibrating its strategy in the context of the U.S. rebalance to Asia. And, although North Korea may be a recalcitrant and counterproductive ally, from many points of view, for Beijing it's the only one its got.”
North Korea owes its existence to China's military support during the Korean War. Just as North Korea was being overrun, Beijing turned the tide of the war by sending an estimated two million soldiers to drive back U.S.-led United Nations troops.
China once characterized its relations with North Korea as “close as lips and teeth.”
But, Pyongyang's belligerence, and pursuit of nuclear weapons, has led Beijing to support U.N. sanctions against it. And, in what some analysts see as a snub, North Korea's young leader Kim Jong Un has yet to visit Beijing since taking his father's place.
Unlike Kim Jong Il who died in 2011, Kim Jong Un does not know how far he can push China's patience, Graham said.
"Kim Jong Il was of course very practiced at being able to co-opt that red line with China but never quite crossing it. This is the risky element to having a young and inexperienced leader who may, through overconfidence, step over that line," he said. "Because, of course, the more that North Korea develops its nuclear and missile capabilities the greater that risk of overconfidence becomes."
Meanwhile, China watchers are looking for any indications of a change in official policy toward North Korea. Baker said they should also keep a close eye on developing relations between Beijing and Seoul.
“You know, ultimately, I think for China the real competition in North Korea is between itself and South Korea," he said. "And so I think that's what we need to really watch, is how this relationship between South Korea and China develops now that we've had this sort of, you know, belligerent confrontation between South and North, where does China really fall? How much does it support North Korea, how much does it support South Korea?”
South Korea and the U.S. are trying to break North Korea's long-held pattern of raising tensions to push for diplomatic and economic concessions, said Baker.
China, meanwhile, continues to urge restraint and a return to the international talks to end North Korea's nuclear program.
The six nation talks involving China, Japan, North and South Korea, Russia and the United States, began in 2003 but ended in 2009 when Pyongyang quit. Efforts to restart negotiations last year were scuttled in December when Pyongyang tested a long-range missile and then in February tested its third nuclear device.
