SEOUL >> Two weeks after North Korea shelled a South Korean island, the rivals are still trading threats of attacks and counterattacks. Tensions remain at their highest in more than a decade, and though neither side is backing down, all-out war is unlikely.

Spooked by the assault that killed four people, South Korea threatened airstrikes if hit again, ordered a doubling of troops on front-line islands and revamped rules of engagement to allow for a more forceful response to future provocations.

And on Wednesday, the top American military officer stood by his South Korean counterpart’s side in Seoul pledging “unquestioned” support for the ally. He pointedly warned Pyongyang to stop its “belligerent, reckless behavior” even as the North continued carrying out live-fire drills from its shores.

On the peninsula still technically at war and divided by only a narrow no-go zone where hundreds of thousands of troops stand ready, there are fears that even a small spark could light the powder keg and send the Koreas back into the war they closed with only a truce in 1953.

Their navies spar from time to time along their maritime border, a line the North Koreans dispute. But the Nov. 23 shelling of the South Korean island of Yeonpyeong was the first to target a civilian area. Two construction workers and two marines were killed.

North Korea expressed regret for the civilian deaths, but blamed the South Korean military for using the residents as “human shields” after Pyongyang warned the South to call off naval artillery drills carried out in the hours before the attack.

And in the days since, both militaries have continued to conduct live-fire shooting exercises that have kept the region on edge.

“The fact that both North and South are having to prove themselves militarily and conduct live-fire tests very close to each other’s borders just increases the likelihood that there could be an errant shell or just a war of nerves that could lead to crossing the line once again,” said Peter Beck, a research fellow at Keio University in Tokyo. “Now that the North has done it once, it’s not going to surprise me if they do it again.”

Still, the doomsday scenario of all-out war across the world’s most militarized border is unlikely, he and other experts said.

South Korea’s moves to bolster its military readiness since the attack reduce the risk of the outbreak of a full-fledged war, said Daniel Pinkston, a Seoul-based analyst with the International Crisis Group think tank.

North Korea is rich in manpower but poor in hardware and, he said, it knows that “further provocation will come at a cost.”

The North’s game has always been to provoke just enough to be able to extract what it needs from the South and the rest of the world.

Since 2003, Pyongyang had been engaged in negotiations with five other nations to dismantle its nuclear program in exchange for fuel oil and other concessions.

After backing out of that deal last year, North Korea — struggling to feed its people, slapped with sanctions — has been looking for a way back to the negotiating table. Seoul and Washington, however, say giving into Pyongyang’s ploys would only reward bad behavior and have resisted restarting the talks.

Complicating matters is that North Korea is handling a sensitive transfer of power from leader Kim Jong Il to his young, untested son. While that uncertainly may make North Korea more unpredictable, it also means it craves stability more than ever.

“North Korea appears likely to carry out reckless provocations as part of heir apparent Kim Jong Un’s move to secure military allegiance, but it would not stage an all-out war” that would cause instability and hurt the North’s succession plan, said Jeon Kyong-mann, an analyst at the state-run Korea Institute for Defense Analyses in Seoul.

Adm. Mike Mullen, chairman of the U.S. Joint Chiefs of staff, said his visit Wednesday was meant to make sure North Korea knows exactly how costly further provocations would be.

The U.S. and South Korea aim to “send a very strong message to North Korea and to whoever might be supporting North Korea that these actions must stop. The goal clearly is to have a deterrent effect so that all-out war never occurs.”

That message was also for China. The neighbor is the North’s only major ally and a crucial source of aid, but has so far been unwilling to push Pyongyang too far. Mullen said it was time for Beijing to enter the fray in a serious way.

“China has unique influence. Therefore, they bear unique responsibility,” Mullen said at a joint news conference with his South Korean counterpart, Gen. Han Min-koo. “Now is the time for Beijing to step up to that responsibility and help guide the North.”

The Korean peninsula technically remains in a state of war because a peace treaty was never signed. Decades later, the two Koreas are divided by the world’s most heavily fortified border, and the U.S. keeps 28,500 troops in the South.

However, North Korea disputes the maritime border drawn by U.N. forces in 1953, and considers the waters around the front-line islands, including Yeonpyeong, its territory.

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Associated Press writer Kim Kwang-tae contributed to this report.