By Foster Klug and Hyung-Jin Kim
North Korea has started loudspeaker broadcasts against the South on their shared border, the South's Yonhap News Agency says.
The move is an apparent countermeasure against Seoul's propaganda loudspeaker campaign.
Taking a step that has angered North Korea in the past, Seoul decided to resume its propaganda broadcasts against Pyongyang earlier on Friday (local time) after North Korea this week announced that it had successfully tested a hydrogen nuclear device.
The broadcasts will draw a furious response from North Korea, which considers them an act of psychological warfare.
Pyongyang is extremely sensitive to any outside criticism of the authoritarian leadership of Kim Jong-Un, the third member of his family to rule.
When South Korea briefly resumed propaganda broadcasts in August after an 11-year break, Seoul says the two Koreas exchanged artillery fire, followed by threats of war.
South Korea's Yonhap news agency reported that frontline troops, near 11 sites where propaganda loudspeakers started blaring messages at noon, were on highest alert.
Yonhap said Seoul had deployed missiles, artillery and other weapons systems near the border to swiftly deal with any possible North Korean provocation.
South Korea's Defence Ministry couldn't confirm the reports and its military banned foreign media from the border ahead of the broadcasts.
The broadcasts include popular Korean pop songs, world news and weather forecasts as well as criticism of the North's nuclear test, its troubled economy and dire human rights conditions, according to Seoul's Defence Ministry.
Included are songs by a young female singer, IU, whose "sweet, girlish voice" might be aimed at North Korean soldiers deployed near the border.
It may take weeks or longer to confirm or refute the North's claim that it successfully tested a hydrogen bomb, which would mark a major and unanticipated advance for its still-limited nuclear arsenal.
Also on Friday, South Korea was to announce the results of its first round of investigations of samples collected from sea operations to see if radioactive elements leaked from the North's test, according to the Korea Institute of Nuclear Safety.
US President Barack Obama has spoken to South Korean President Park Geun-hye and Japanese Prime Minister Shinzo Abe and reaffirmed the "unshakeable US commitment" to the security of the two Asian allies.
Separate statements from the White House said Obama and the two Asian leaders also agreed to countries "agreed to work together to forge a united and strong international response to North Korea's latest reckless behaviour."
South Korean and US military leaders also discussed the deployment of US "strategic assets" in the wake of the North's test, Seoul's Defence Ministry said on Thursday.
The North's claim of a successful test drew extreme scepticism abroad.
An early analysis by the US government was "not consistent with the claims that the regime has made of a successful hydrogen bomb test," White House spokesman Josh Earnest said.