South Korean officials cranked up banks of loudspeakers near the demilitarized zone with North Korea, blaring pop music, news reports and other information into the isolated country.
Among the tidbits drifting north: "Bang Bang Bang,"
a song by K-pop chart toppers Big Bang.
In August, similar propaganda broadcasts led to an exchange of artillery fire
between the two nations.
North Korea fears such broadcasts even more than crippling economic sanctions, said Chun Yungwoo, a senior adviser for the Asan Institute, told CNN.
"The most dangerous virus that could destroy the North Korean regime or the foundations, ideological theocratic foundations of the North Korean regime, is the truth about North Korea, truth about the outside world," he said.
'Declaration of war'
No hostilities have been reported as a result of the current broadcasts, which North Korea has previously described as tantamount to a declaration of war.
And South Korean officials batted reports that North Korea was blaring broadcasts of its own.
But the lack of fisticuffs or other response from the North did little to tamp down worry about Pyongyang's nuclear program and its most recent test -- whatever it was.
Experts have cast doubt on the North's claim that it tested a hydrogen bomb. Among other things, the blast detected by seismic monitors was much smaller than would have been expected with a powerful hydrogen bomb, they say.
A U.S. Air Force plane equipped with special sensors to detect nuclear blasts began operating in the region Thursday, a U.S. official told CNN Friday. Results won't be known immediately as it will take some time for labs to analyze the results, the official said.
Ground monitoring stations belonging to the United States and other nations are analyzing samples, according to the official.
Regardless of exactly what it was, what is widely regarded as North Korea's fourth test of a nuclear weapon remained a worrying development.
British Foreign Secretary Phillip Hammond was just one of many urging deep breaths all around.
"North Korea acts in a totally irresponsible and provocative way," he said Friday. "So I can entirely understand the pressure South Korea feels to respond. But, we have to be bigger than the North Koreans and I would urge South Korea and other like-minded countries in the region to exercise restraint. We know that responding in this way is simply rising to the bait North Korea is presenting to us."
China -- North Korea's most important ally -- said North Korea should "avoid taking action" that would make the situation worse.
And in Japan, the National Assembly passed a resolution on Friday deploring the test and calling for additional U.N. and Japanese sanctions, the Japanese Yonhap News Agency reported.
North Korea, which first tested a nuclear weapon in May 2009, has been under U.S. sanction over its nuclear program since at least 1992. The United Nations joined in 2006.
North Korea has said it needs a strong nuclear deterrent to keep the United States from invading. The United States and its allies have said North Korea's nuclear tests are provocative, destabilizing and dangerous.