SEOUL: South Korea will resume loudspeaker propaganda broadcasts towards North Korea at the inter-Korean border from Friday in response to North Korea's fourth nuclear test, South Korea's presidential office said on Thursday.

The North's nuclear test was a "grave violation" of an August agreement reached between the two Koreas to ease tension and improve ties, a senior presidential national security official, Cho Tae-yong, said in a statement.

"Our military is at a state of full readiness, and if North Korea wages provocation, there will be firm punishment," Cho said.

North and South Korea traded artillery fire in August after Pyongyang demanded a halt to the loudspeaker broadcasts. The rivals negotiated an end to the standoff with an agreement to improve ties.