South Korea has resumed propaganda broadcasts via loudspeakers into North Korea, a step that has angered the country in the past.
Friday's moves came as the international community struggled to find common ground on how best to penalise North Korea following its announcement two days ago that it had successfully testedits first hydrogen bomb.
The loudspeaker broadcasts began at noon local time (03:00 GMT) and an official said the military heightened the level of alert around the locations where the propaganda was being broadcast.
"We're putting out critical messages about Kim Jong-un's regime and its fourth nuclear test, saying North Korea's nuclear weapons development is putting its people in more difficult times economically," a military official said.
The resumption of the cross-border broadcasts, which include a mix of everything from K-pop and weather forecasts to snippets of news and critiques of the North Korean regime, revives psychological warfare tactics that date back to the 1950-53 Korean War.
Their use during a flare-up in cross-border tensions last year angered North Korea, which, at one point, threatened artillery strikes against the loudspeaker units unless they were switched off.
The South finally ended the broadcasts after an agreement was reached in August to de-escalate a situation that had brought the two rivals to the brink of an armed conflict.
Al Jazeera's Scott Heidler, reporting from Paju in South Korea, said that the North viewed this as an act of war and a distinct violation.
"South Korea has said it will continue this indefinitely and we haven't yet seen direct retaliation from the North Koreans."
North Korea's test prompted the UN Security Council to discuss possible sanctions as world leaders sought to build a consensus on an appropriate response.
On Thursday President Barack Obama spoke with the leaders of the two main US allies in Asia - and North Korean neighbours - South Korea and Japan.
The three countries, who have long sought to project a united front against the North Korean nuclear threat, agreed to work together at the UN to secure the strongest possible Security Council resolution.
North Korea, meanwhile, has said virtually nothing since its TV broadcast at noon on Wednesday announcing the "world startling event" of its latest test.
The test, personally ordered by leader Kim Jong-un, was of a miniaturised H-bomb, North Korea said, adding that it had now joined the ranks of "advanced nuclear nations".