SEOUL — The chairman of the U.S. Joint Chiefs of Staff said Wednesday the U.S. would not stand in the way of a strong South Korean response to further North Korean aggression, and had not asked the South to rule out airstrikes — an action that could dramatically escalate the simmering conflict between the two Koreas.

“South Korea is a sovereign nation that has every right to protect its people and to respond as it sees fit in order to carry out that responsibility,” Adm. Mike Mullen said following a meeting with top South Korean defense officials at the Ministry of National Defense. “They also have the right to choose the method with which they respond.”

After North Korea’s bombardment of South Korea’s Yeonpyeong Island two weeks ago, South Korean officials have promised to respond to further North Korean provocations with greater force. During his confirmation hearings last week, South Korea’s defense minister, Kim Kwan-jin, went as far as saying the South would opt for airstrikes on North Korea if another attack is launched.

At a press conference following Wednesday’s meeting, Mullen and his South Korean counterpart, Gen. Han Min-koo, said the two sides were exploring options aimed at avoiding an all-out war with the North. Both countries agreed primarily to “refine” their plans for responding to a North Korean attack, according to a news release issued after the meeting.

Han said North Korea’s artillery shelling of Yeonpyeong had been uncharacteristically bold, indicating that the South and the U.S. needed to develop joint plans for responding to future attacks. South Korea — not the U.S., which has 28,500 troops stationed there — is supposed to respond to peacetime violations of the armistice that ended the Korean War.

“Their provocations are going beyond our imagination,” Han said, adding that the North would “pay a very deep price” for further attacks.

Mullen also blasted China for its apparent unwillingness to curb North Korea’s provocations toward the South.

In one of the most scathing U.S. critiques of China since last month’s attack on the South Korean island, Mullen said China has a “unique responsibility” to curb North Korea because of its influence over the reclusive nation, which last month also revealed a new uranium enrichment facility.

“The Chinese have enormous influence over the North, influence that no other nation on earth enjoys,” said Mullen, speaking at the Ministry of National Defense after meeting with South Korean defense officials. “And yet, despite a shared interest in reducing tensions, they appear unwilling to use it.

“Even tacit approval of Pyongyang’s brazenness leaves all their neighbors asking, ‘what will be next?’ ”

The attack on Yeonpyeong killed four people. It was the first North Korean attack on civilians since the Korean War, and came eight months after the sinking of a South Korean warship.

Mullen rebuffed China’s offer to host an emergency meeting of the countries participating in the six-party talks, aimed at curbing North Korea’s nuclear program.

“We appreciate Beijing’s offer to propose an emergency six-party gathering,” he said. “But as [Secretary of State Hillary Clinton] made clear, we first need an appropriate basis for the resumption of talks.”

He also said the U.S. will not return to direct talks with North Korea until it ceases its “illegal, ill-advised and dangerous behavior. I do not believe we should continue to reward that behavior with bargaining or new incentives.”

Only hours before Mullen was to speak, there were reports that North Korea fired artillery shells into its water near the disputed western border with South Korea. None of the shells fell crossed the border into South Korean waters and the incident is under investigation, South Korean officials told Bloomberg News.

Han said he did not know if Wednesday’s artillery firing was meant to disrupt the meeting with Mullen.

John Delury, a professor at Yonsei University in Seoul, said the six-party talks, last held in 2007, act as a “shock absorber” to ease tensions on the peninsula by giving participants — the U.S., South Korea, North Korea, China, Russia and Japan — a chance to discuss their concerns.

“Without a process in place to put this whole situation on a different track, I think it’s a matter of time before there’s further conflict,” he said. “Now there’s no place to go to talk about issues that may instead be communicated through force.”

Mullen said he and Han discussed ways to counter what he described as a “rapidly-evolving threat” on the Korean peninsula that falls in line with the current defense alliance between the two countries. He did not elaborate on those plans, but said U.S. Forces Korea commander Gen. Walter Sharp will work closely with South Korea to develop further plans and exercises.

Though South Korean leaders have been widely criticized at home for a sluggish response to the North Korean shelling, Mullen praised South Korea for its restraint in responding to the attack on Yeonpyeong.

Mullen’s visit to Seoul is the latest move in a flurry of diplomatic meetings and military exercises held since the Yeonpyeong attack. Clinton met with her Japanese and South Korean counterparts in Washington on Monday in a show of unity that ended in the three diplomats calling for greater cooperation from China and Russia.

He called for more trilateral consultation among the U.S., South Korea and Japan, saying “this is a critical region where all regional players matter.”

Mullen will travel to Tokyo on Wednesday evening to meet with Japanese defense officials, whom he praised for their cooperation in defending against the North Korean threat.

“I only wish China were as helpful,” he said.

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