President Barack Obama on Tuesday pledged the United States would stand "shoulder to shoulder" with South Korea after what the White House branded a provocative, outrageous attack by North Korea on its neighbor. Its options limited, the U.S. sought a diplomatic rather a military response to one of the most ominous clashes between the Koreas in decades. "South Korea is our ally. It has been since the Korean war," Obama said in his first comments about the North Korean shelling of a South Korean island early Tuesday. "And we strongly affirm our commitment to defend South Korea as part of that alliance." Working to head off any escalation, the U.S. did not reposition any of its 29,000 troops in the South or make other military moves after North Korea fired salvos of shells into the island, setting off an artillery duel between the two sides.