South Korea has resumed propaganda broadcasts via loudspeakers into North Korea, a step that has angered the reclusive 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 tested its first hydrogen bomb.
The cross-border broadcasts began at noon local time (03:00 GMT) and an official said the military had heightened the level of alert around the locations of the loudspeakers.
"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.
North Korea, in return, started its own loudspeaker broadcasts on the shared border, the South's Yonhap News Agency said on Friday.
The resumption of the South Korean broadcasts, which include a mix of everything from K-pop and weather forecasts to snippets of news and critiques of the North Korean government, 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 ended the broadcasts after an agreement was reached in August to de-escalate a situation that had brought the Korean Peninsula to the brink of an armed conflict.
No 'direct retaliation yet'
Al Jazeera's Scott Heidler, reporting from Paju in South Korea on Friday, said the North viewed the resumption of the broadcasts as an act of war and a distinct violation.
"There are 10 different locations where the broadcast is being done from as well as mobile units," he said.
"South Korea has said it will continue this indefinitely, and we haven't yet seen direct retaliation from the North Koreans."
North Korea's claimed hydrogen-bomb test has prompted the UN Security Council to discuss possible sanctions as world leaders seek to build a consensus on an appropriate response.
ANALYSIS: Breaking down North Korea's H-bomb test claim
President Barack Obama spoke on Thursday with the leaders of the two main US allies in Asia - and North Korean neighbours - South Korea and Japan.
The three countries, which 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 has said virtually nothing since its TV broadcast at noon on Wednesday announcing the "world startling event" of its latest test.
The test was personally ordered by leader Kim Jong-un and was of a miniaturised H-bomb, the country said.
North Korea further said it had now joined the ranks of "advanced nuclear nations".
Source: Al Jazeera and agencies