North Korea fires on South Korea

President Obama and his South Korean counterpart Lee Myung-bak have pledged to hold joint military exercises and enhanced training after North Korea’s military forces fired artillery against a South Korean island, prompting the South’s forces to return fire.

Mr. Obama called Mr. Lee late on Tuesday night to assure him that the U.S. “stands shoulder to shoulder with our close friend and ally” South Korea, the White House said in a statement.

The two presidents agreed to “continue the close security cooperation between our two countries, and to underscore the strength of our alliance and commitment to peace and security in the region,” it added.

Mr. Obama strongly condemned the attack by the North on Yeonpyeoung island that claimed the lives of two South Korean marines and wounded dozens of others. It also set fire to more than 60 buildings and pushed the two Koreas to the brink of war.

Mr. Obama said the North must “stop its provocative actions, which will only lead to further isolation, and fully abide by the terms of the Armistice Agreement and its obligations under international law,” the White House said.

Earlier in the day, Mr. Obama met with senior aides to discuss how to respond to the latest crisis over North Korea.

The president was briefed by National Security Adviser Tom Donilon, Secretary of State Hillary Rodham Clinton and Defense Secretary Robert M. Gates, along with U.S. Ambassador to the United Nations Susan E. Rice, Joint Chiefs of Staff Chairman Adm. Mike Mullen, and Army Gen. Walter Sharp, commander of U.S. Forces Korea.

Mr. Obama stressed during the meeting that the U.S. has “unshakable support” for South Korea and “discussed ways to advance peace and security on the Korean Peninsula going forward,” according to the White House.

The aircraft carrier strike group led by the USS George Washington left its home port of Yokosuka on Monday. A Pacific Fleet spokesman in Hawaii said the strike group will remain in the region and await any further tasking from senior leaders.

At the Pentagon, Mr. Gates telephoned his South Korean counterpart, Defense Minister Kim Tae-young, and said the United States regarded the North Korean artillery attack as a violation of the 1953 armistice agreement that ended the Korean War.

The armistice agreement did not lead to a peace treaty and thus both nations remain technically at war.

Pentagon Press Secretary Geoff Morrell told reporters that Mr. Gates expressed appreciation for “restraint” shown by Seoul over the attack.

The artillery strike, which began at midafternoon Tuesday on the South Korean island of Yeonpyeong off the west coast, killed two South Korean marines and injured dozens of others. It also set fire to more than 60 buildings and pushed the two Koreas to the brink of war.

Dozens of artillery rounds were fired in three separate barrages. South Korea fired about 80 artillery rounds in retaliation.

The island is two miles south of the Northern Limit Line, the disputed sea border that the North does not recognize. South Korean military installations and a small civilian population are on the island.

The attack is the latest provocation by the economically strapped communist regime in Pyongyang and followed the sinking in March of the South Korean warship Cheonan in the same region. An international investigation later concluded that a North Korean torpedo, probably fired from a mini-submarine, caused the sinking and the deaths of 46 sailors.

Tensions also have been raised over North Korea’s once-covert uranium enrichment program that was shown recently to three visiting American nuclear specialists.

North Korean state radio interrupted broadcasting at 7:28 p.m. local time for a statement from the Korean People’s Army Supreme Command that denounced the South Korean military for holding military exercises.

The statement claimed South Korea had launched artillery shells in North Korean waters and that its firing was designed to counter the attack. The statement said its artillery firing was an “immediate and powerful physical strike” and warned that a “merciless” counterstrike would follow any further maritime border violations.

A diplomat in Washington familiar with the region said the North Koreans fired the artillery in response to South Korean military exercises near the island. The diplomat said the exercises did not violate North Korean sovereignty because they were held on the South Korean side of the line separating the two countries.

“Our military fired artillery toward south of Yeonpyeong Island,” the diplomat said. “Therefore, North Korean assertions that their actions were in response to our military exercises were totally groundless.”

South Korea put its military forces on high alert after the incident, and South Korean President Lee Myung-bak convened an emergency Cabinet meeting to discuss a response to the attack. Mr. Lee said an “indiscriminate attack on civilians can never be tolerated.”

“Enormous retaliation should be made to the extent that [North Korea] cannot make provocations again,” Mr. Lee said.

In Seoul, a spokesman for Mr. Lee said the attack on Yeonpyeong was an “indisputable armed provocation” against South Korea.

“Such actions will never be tolerated,” the spokesman said, adding the North must take full responsibility for the incident.

Mr. Obama was awakened by Mr. Donilon about 4 a.m. and notified of the incident.

Later Tuesday morning, White House press secretary Robert Gibbs called on the North to “halt its belligerent action” and fully abide by the terms of the 1953 Armistice Agreement.

Bill Burton, White House deputy press secretary, told reporters aboard Air Force One en route to Kokomo, Ind., that Mr. Obama was “outraged” by the North’s actions.

In New York, U.N. Secretary-General Ban Ki-moon said he was deeply concerned about the escalation of tension on the Korean Peninsula and described the attack as one of the “gravest incidents since the end of the Korean War,” said Mr. Ban’s spokesman Martin Nesirky.

Mr. Ban, a former South Korean foreign minister, condemned the attack and called for immediate restraint.

Tuesday’s attack was the latest in a string of incidents. In August, the North fired 110 artillery shells near Yeonpyeong and another island.

It also was disclosed that North Korea is building a light-water nuclear reactor, an indication that concerns about a uranium enrichment program were borne out.

Stephen W. Bosworth, the U.S. special representative for North Korea policy, said while this was not reason enough to reassess the entire U.S. strategy toward North Korea and its nuclear programs, it is a very serious development.

“We are not calling into question our overall approach to this, which is an approach based on a multilateral effort, close coordination with the other four countries involved, and a commitment to dialogue, and a continued pursuit of the implementation of the joint statement of September 2005,” Mr. Bosworth said in Beijing. “But we are very concerned as to the sincerity of the [North’s] approach to this.”

The George W. Bush administration took North Korea off the list of state sponsors of terrorism in 2008 in exchange for an unfulfilled promise by Pyongyang to dismantle its nuclear-weapons program.

Sung Kim, the U.S. special envoy to the six-party talks to denuclearize the Korean Peninsula, said the development at the Yongbyon nuclear facility was a cause for concern.

“We should be worried. … This revelation of a facility in Yongbyon may be new, but this is an issue we have followed for some time, and have been concerned about for quite some time,” Mr. Kim said at a discussion at the Center for Strategic and International Studies (CSIS) on Monday.

The developments also have resulted in calls on China to condemn the actions of its communist ally.

Mr. Kim, the U.S. special envoy, said Beijing has a “special responsibility” to condemn the North’s actions owing to its “special relationship” with Pyongyang.

Mr. Bosworth discussed the attack on Yeonpyeong with Chinese officials in Beijing on Tuesday.

“I think we both share the view that such conflict is very undesirable,” Mr. Bosworth said.

He said the U.S. and Chinese sides “strongly believe that a multilateral, diplomatic approach is the only way to realistically resolve these problems.”

China appealed to the Koreas to remain calm and “to do more to contribute to peace and stability on the peninsula,” Foreign Ministry spokesman Hong Lei said.

The White House said the U.S. will work with the international community to strongly condemn the “outrageous action by North Korea, and to advance peace and security in the region.”

The North-South border is one of the most heavily militarized borders in the world.

The North deploys approximately 65 percent of its military units, and up to 80 percent of its estimated aggregate firepower, within 60 miles of the demilitarized zone (DMZ), according to an estimate by the International Institute for Strategic Studies.

House Speaker-designate John A. Boehner, Ohio Republican, said in a statement that North Korea is an “unstable, aggressive regime” and he joined Mr. Obama in condemning the attack by the North.

• This article is based in part on wire service reports.