US Defense Secretary Leon Panetta discussed "immediate coordinated actions" with his South Korean counterpart on Wednesday, after North Korea staged a third nuclear test, a Pentagon spokesman said.
Panetta spoke with South Korea's Defense Minister Kim Kwan-Jin one day after Pyongyang triggered international outrage bay carrying out its latest nuclear detonation in defiance of UN Security Council resolutions.
Pentagon press secretary George Little said in a statement that the allies discussed "immediate coordinated actions and continuing collaboration in the wake of North Korea's highly provocative nuclear test."
Panetta reaffirmed Washington's pledge to defend South Korea against attacks -- "including our extended deterrence commitment" -- a reference to America's own nuclear arsenal.
The Pentagon chief stressed the importance of imposing additional sanctions on North Korea, the statement said.
US and South Korean monitors said the underground nuclear test was much more powerful than North Korea's previous tests in 2006 and 2009.
The UN Security Council, including China, united on Tuesday to condemn the test and accuse North Korea of a "grave violation" of UN resolutions.
President Barack Obama, in his State of the Union address Tuesday, vowed to take "firm action" against North Korea.