(For a Take A Look on Korea situation [ID:nKOREA])
* White House wants North to halt "belligerent" activity
* Obama to talk to S.Korean president-W.House -Pentagon
* No U.S. forces involved in response to attack (Adds White House meeting, McCain statement)
By Phil Stewart and Andrew Quinn
WASHINGTON, Nov 23 (Reuters) - The United States urged restraint on Tuesday following a North Korean artillery attack on South Korea and vowed to forge a "measured and unified" response with major powers including China.
North Korea fired several dozen artillery shells at a South Korean island in one of the heaviest bombardments of the South since the Korean War ended in 1953, sharply increasing tensions on the divided peninsula. [ID:nL3E6MN0SQ]
South Korea warned North Korea of "enormous retaliation" if it took more aggressive steps. But the United States, which has around 28,000 troops stationed in South Korea, played down the chances of any immediate U.S. military action to deter the reclusive state.
"We're still monitoring the situation and talking with our allies. But I wouldn't say there's anything that's been initiated as a result of the incident," said Pentagon spokesman Colonel Dave Lapan.
State Department spokesman Mark Toner said the United States was seeking a unified diplomatic front with North Korea's neighbors including China, Pyongyang's sole remaining major backer which has in the past resisted international efforts to get tough with its isolated ally.
"We're not going to respond willy nilly," he said.
The White House strongly condemned the attack. U.S. President Barack Obama was awoken at 3:55 a.m. for an emergency briefing and was outraged over the strike, the White House said. He was due to speak with South Korean President Lee Myung-bak, White House spokesman Bill Burton said.
"North Korea has a pattern of doing things that are provocative. This is a particularly outrageous act," Burton said aboard Air Force One as Obama headed to Indiana to visit an auto plant.
The White House said Obama's top national security aides including Secretary of State Hillary Clinton and Defense Secretary Robert Gates were due to meet on Tuesday discuss the situation.
The South fired back after Tuesday's attack and sent fighter jets to the area, but no U.S. forces were involved in the South's response, a U.S. official said.
Global stock markets fell in reaction to the escalting tensions. In the United States, major stock indexes such as the Dow Jones Industrial Average fell about 1.5 percent while an investor flight to safety pushed up gold and the U.S. dollar [ID:nN23162064]
ATTACK FOLLOWS NUCLEAR REVELATIONS
The artillery attack posed the second test in three days of Washington's vow that it will not reward what it deems bad behavior with diplomatic gestures, like resuming aid-for-disarmament talks.
The attack followed revelations over the weekend of a uranium enrichment facility -- a second source of atomic bomb material in Pyongyang's nuclear program.
In Washington, Republican lawmakers took the lead in calling for a tougher approach to North Korea.
"Two decades worth of attempts to appease this North Korean regime have failed, and further attempts to do so will meet with the same result," Senator John McCain, the Republican's presidential candidate in 2008, said, calling on Beijing to "play a more direct and responsible role in changing North Korea's reckless behavior."
Stephen Bosworth, the U.S. envoy on North Korea who was in Beijing for talks, said all sides agreed restraint was needed: "I expressed to them the desire that restraint be exercised on all sides and I think we agree on that."
Analysts said the North may be again pursuing a strategy of calculated provocations to wrest diplomatic and economic concessions form the international community.
Asked about the North's motives, Pentagon Press Secretary Geoff Morrell said: "I don't know. This is an extremely unpredictable government in Pyongyang and they do things that you could not possibly have predicted in a rational world."
Morrell also appeared to play down the possibility of more sanctions, telling MSNBC television: "It's hard to pile more sanctions upon the North than are already there."
(Additional reporting by Caren Bohan aboard Air Force One, Alister Bull, Steve Holland, Doina Chiacu and Missy Ryan in Washington; Editing by Doina Chiacu and Cynthia Osterman)
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