WASHINGTON - As South Korea threatened retaliation for North Korea's deadly shelling of a South Korean island, U.S. officials Tuesday scrambled to avert any catastrophic escalation of hostilities following one of the most serious confrontations on the Korean peninsula in decades.

The shelling - which killed two soldiers and injured 19 people, including three civilians - sent refugees fleeing the western island of Yeonpyeong as the South Korean government ordered its air force on high alert and declared that the secretive North Korean regime would face "stern retaliation" if it launched further attacks.

Condemnation of the North came swiftly from overseas capitals. President Barack Obama was "outraged," an aide said, saying Pyongyang was "an ongoing threat that needs to be dealt with." The White House called on Pyongyang to end "its belligerent action." The Obama administration sought to build diplomatic pressure on the North by enlisting the aid of China, which provides vital energy assistance and other aid to the impoverished regime. U.S. officials and allies began trying to round up support for a United Nations Security Council statement that would swiftly condemn Pyongyang's action, diplomats said.

A number of high-ranking members of Congress called on China to exert stronger influence with the North. House Foreign Affairs Chairman Howard Berman (D-Calif.) urged Beijing to "immediately suspend its economic and energy assistance to show Pyongyang that its aggression has consequences."

The South Korean military was conducting drills near the island, which is close to the North-South border, when the North opened fire about 2:30 p.m. local time Tuesday. Pyongyang had sent messages to Seoul that it considered the exercises "preparation for an invasion." The killings of soldiers and the attack on civilians puts South Korean President Lee Myung-bak in the position of having to respond strongly while avoiding dangerous escalation, analysts said.

Senior U.S. officials, including Defense Secretary Robert M. Gates and Secretary of State Hillary Rodham Clinton, met at the White House Tuesday to discuss the incident. Obama was to call the South Korean president late Tuesday to express firm U.S. commitment to South Korean security, officials said.

The North's attack followed the disclosure over the weekend that the North is building a uranium enrichment plant at its nuclear site in Yongbyon, suggesting that it is seeking a second method of building nuclear weapons.

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