South Korea on Thursday said it will resume cross-border propaganda broadcasts after its northern neighbor claimed it successfully tested its first hydrogen bomb.
Seoul said the broadcasts, which North Korea considers an act of war, will begin Friday.
Cho Tae-yong, deputy chief of South Korea's presidential office of national security, told reporters that the test is a "grave violation" of a deal reached between the two neighbors in August to defuse tensions after a landmine at the border injured two of the South's soldiers.
The deal included a pledge by the South to stop anti-North broadcasts along the border. Further high-level talks last month ended without any agreement.
Cho said South Korean troops are combat-ready and will "sternly retaliate" if provoked by the North, the Yonhap news agency reported.
South Korean and American military officials discussed the deployment of U.S. "strategic assets" following the test, the South's Defense Ministry said Thursday.
On Wednesday, the U.N. Security Council held an emergency meeting on the reported test, which was strongly condemned by the international community, and pledged to pursue new sanctions.
North Koreas's announcement of the test came soon after a magnitude-5.1 earthquake was reported 30.4 miles from the city of Kilju, North Korea, where the country's Punggye-ri nuclear test site is located.
The secretive state conducted nuclear tests there in 2006, 2009 and 2013.
The North's claim was met with widespread skepticism. White House press secretary Josh Earnest said the initial analysis by U.S. intelligence agencies is "not consistent with North Korean claims of a successful hydrogen bomb test."
A hydrogen bomb is larger and more complex than an atomic bomb. South Korea's intelligence agency said the power of the explosion was smaller than what a hydrogen, or thermonuclear, bomb would produce. It could be several weeks before a conclusive determination is made.