SEOUL, South Korea — As world leaders debated how to punish North Korea’s claim of a fourth nuclear test, South Korea retaliated by broadcasting anti-Pyongyang propaganda across the rivals’ tense border Friday on what was believed to be Kim Jong Un’s birthday.

North Korea considers such broadcasts to be an act of psychological warfare and likely will have a furious response. Pyongyang is extremely sensitive to any outside criticism of the authoritarian leadership of Kim, the third member of his family to rule the country. 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 loudspeakers started blaring propaganda 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 Defense Ministry did not immediately confirm the reports.

The North Korean response could be harsh because of the high emotions surrounding the likely birthday of Kim, who is believed to be in his early 30s. North Korean military forces often compete to show their loyalty to the leader. The North’s state media has yet to mention Kim’s birthday or South Korea’s loudspeaker campaign.

The broadcasts came as world powers sought to find other ways to punish the North for conducting what it said was its first hydrogen bomb test Wednesday.

U.S. Secretary of State John Kerry urged China, the North’s only major ally and its biggest aid provider, to end “business as usual” with North Korea.