Tensions on the Korean Peninsula have increased since the December death of North Korean leader Kim Jong Il.
North Korea had agreed to a moratorium on nuclear work and long-range missile deployments in exchange for substantial food assistance from the United States. That deal collapsed last month when North Korea tried to send a satellite into orbit using a long-range rocket.
The failed deployment coincided with commemorations marking the 100th birthday of North Korean founder Kim Il Sung. South Korea had expressed concern about the lavishness of the celebrations, given concerns about famine in North Korea. That provoked a sharp response from Pyongyang.
U.S. and South Korean militaries had conducted military exercises in the region. Mark Toner, a spokesman for the U.S. State Department said Washington was committed to Seoul's defense.
"We are strongly in support of the defense of South Korea, and that goes without saying that we seek to work productively and constructively with South Korea on meeting their security needs," he said.
The U.S.-South Korean military partnership is nearly 60 years old.