SEOUL — In one of the most serious escalations of aggression since a 1953 armistice halted the Korean War, North Korea fired dozens of artillery rounds at a small South Korean island Tuesday, killing two South Korean marines and setting fire to dozens of civilian structures. South Korea returned the fire and put its armed forces on “crisis status.”

Officials from Seoul to Washington, already concerned about earlier revelations of a new North Korean uranium-enrichment facility, scrambled to find an appropriate response to the latest provocation from Pyongyang and keep the volatile situation from spinning out of control.

A spokesman for the South Korean Ministry of National Defense said that the U.S. and South Korea had agreed to operate in a “joint crisis management” mode. Meanwhile, South Korean President Lee Myung-bak convened a meeting of top ministers Tuesday evening and ordered missile strikes on the North Korean base that initiated the artillery barrage if Pyongyang showed any hint of further aggression, the Korea Herald reported.

The Obama administration quickly denounced the attack from the “belligerent” North but offered few details regarding a response.

“We are in close and continuing contact with our Korean allies,” White House press secretary Robert Gibbs said in a statement. “The United States is firmly committed to the defense of our ally, the Republic of Korea, and to the maintenance of regional peace and stability.”

No new U.S. military assets were being moved into the region, Pentagon spokesman Col. David Lapan said Tuesday morning, calling it “premature” to say whether the U.S. would be involved in any military response.

About 28,500 American troops are stationed in South Korea, along with thousands of their family members and civilian workers.

South Korea began a major military exercise on Monday in the region where the North Korean attack occurred, and though in previous years the exercise was jointly conducted with the U.S. military, Lapan said the Pentagon believed no American forces were involved this year.

In an MSNBC interview, Pentagon press secretary Geoff Morrell said the incident was alarming, but not out of step with recent provocations from the North.

“We take this very seriously, just as we took the sinking of the Cheonan earlier this year very seriously, in which the North murdered some 40 South Korean sailors,” Morrell said. “It’s hard to pile more sanctions upon the North than are already there, and yet it seems as though they are not fool-proof.”

Tensions have been rising since the attack on the Cheonan, a South Korean ship on patrol in the Yellow Sea near the disputed maritime border between the two countries. A South Korean-led international investigation found that a North Korean torpedo sank the ship, killing 46 sailors. North Korea has repeatedly denied any involvement in the sinking.

By Tuesday evening, officials began to assess possible civilian casualties and damage to buildings on Yeonpyeong, an island of about 1,200 residents in the Yellow Sea, according to a South Korean Ministry of National Defense spokesman. South Korean television broadcasts showed plumes of smoke rising from the island.

North Korea shot dozens of rounds at the island beginning at 2:34 p.m. and South Korea retaliated with 80 shells, the spokesman said. He said the firing continued sporadically after the initial burst of rounds but had stopped by Tuesday night.

Six marines were seriously injured and 10 sustained minor injuries in the bombardment, he said. Officials said they could not comment on the extent of injuries to civilians.

South Korean fighter jets were scrambled to monitor the situation, but had not opened fire.

In the 1 1/2 months since ailing North Korean dictator Kim Jong Il gave his son, Kim Jong Un, the rank of four-star general--anointing him as his chosen successor--the reclusive country has been provoking its southern neighbor.

Last week, North Korea claimed it had built a new uranium-enrichment facility, as part of its effort to expand its nuclear arsenal. And in late October, North Korean troops fired two rounds toward South Korea on the same day U.S. Forces Korea commander Gen. Walter Sharp was visiting another section of the Demilitarized Zone. Sharp also heads the U.N. Command, and would assume command of all U.N. troops in the country during wartime, including those of South Korea.

The top U.S. diplomat on North Korean issues was meeting with Chinese officials in Beijing to discuss North Korea’s recent nuclear revelation when the attacks occurred.

“The subject did, of course, come up in my meetings with the Chinese Ministry of Foreign Affairs, and I think we both share the view that such conflict is very undesirable,” Ambassador Stephen Bosworth, special representative for North Korea policy, said at a news conference. “I expressed to them the desire that restraint be exercised on all sides, and I think we agree on that.”

Asked if Pyongyang’s actions make the resumption of six-party negotiations more difficult to achieve, Bosworth replied, “The resumption of six-party talks has never been an easy process. What we agreed today, in my conversations here, is that from the point of view of China and the United States, we strongly believe that a multilateral, diplomatic approach is the only way to realistically resolve these problems.”

Hours after Tuesday’s skirmish, North Korea’s supreme military command threatened to continue strikes against its rival if it violated their disputed sea border “even 0.001 millimeter,” according to the North’s official Korean Central News Agency, The Associated Press reported.

Sharp canceled an appearance with the foreign press scheduled for Wednesday morning. In a speech at a Tuesday morning ceremony reinstating South Africa as a member of the U.N. Command, Sharp said North Korea “had chosen the way of confrontation and provocation – including launching unprovoked attacks and developing ballistic missiles and nuclear weapons.”

“The Republic of Korea has educated and empowered its citizens while North Korea has starved and abused its people,” he said.

North Korea and South Korea have clashed several times in the Yellow Sea in the past decade near the maritime demarcation line.

The Northern Limit Line, which separates fishing waters between the two countries, was established by the United Nations after the 1950-53 Korean War but has never been recognized by North Korea.

“The North Korean regime is more dangerous than most people realize,” Rep. Ike Skelton, the outgoing chairman of the House Armed Services Committee, said in a statement. “This provocative attack is reprehensible and is in direct violation of the Armistice Agreement. Such behavior is unacceptable.”

Stars and Stripes reporter Kevin Baron contributed to this report from Arlington, Va.

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