U.S. Defense Secretary Chuck Hagel spoke by phone with South Korean Defense Minister Kim Kwan-jin to discuss heightened regional tensions in the wake of North Korea's nuclear test in February.
Pentagon spokesman George Little said Hagel expressed unwavering commitment to regional security.
"The secretary highlighted the steadfast U.S. commitment to the defense of South Korea, including extended deterrence capabilities," Little said.
The U.S. military confirmed Thursday that it sent two B-2 stealth bombers from a Missouri airbase to South Korean air space for what's billed as a "long-duration, round-trip training mission."
"The United States is steadfast in its alliance commitment to the defense of the Republic of Korea, to deterring aggression, and to ensuring peace and stability in the region," a statement from the U.S. Forces Korea said. "The B-2 bomber is an important element of America's enduring and robust extended deterrence capability in the Asia-Pacific region."
North Korea recently sounded air-raid sirens in Pyongyang in response to the presence of B-52 bombers in the region.