UPDATE: 4:15 a.m. EST — North Korea began loudspeaker broadcasts against South Korea Friday after Seoul resumed its propaganda loudspeaker campaign, Reuters reported, citing Yonhap news agency.

Original story:

South Korea said Friday that its military has heightened the alert level on its border with North Korea as propaganda broadcasts begin in response to Pyongyang’s claim to have successfully tested a hydrogen bomb, according to reports. 

Tensions between the two Koreas escalated Wednesday after North Korea carried out an alleged hydrogen bomb test — a move that has been criticized by the international community. The US and United Nations have also announced that they're working on sanctions against the North as a punitive measure.

"We are 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, according to Reuters.

South Korea's Yonhap news agency reported that Seoul had deployed missiles, artillery and other weapons systems near the border to retaliate against any possible provocations from North Korea, according to the Associated Press.

According to reports, the propaganda broadcasts from loudspeakers could likely draw an angry response from North Korea, which is extremely sensitive to outside criticisms. The propaganda broadcasts coincide with the North Korean leader's 32nd birthday celebrations.

U.K. urged South Korea to show restraint as propaganda broadcasts against the North began."We urge South Korea to exercise restraint," British Foreign Secretary Philip Hammond said during a visit in Japan, according to Reuters. "It is simply rising to the bait."

Last August, the two countries reportedly exchanged artillery fire over the issue before reaching a deal that ended the broadcasts and pledged other measures to ease tensions between the two Koreas.