North Korean leaflets criticizing the South were spotted for a second consecutive day in South Korea's border towns Thursday as the rivals escalated their psychological warfare across the border in the wake of the communist nation's recent nuclear test.

"We detected the North Korean military spreading anti-South leaflets again last night and early in the morning today," Joint Chiefs of Staff spokesman Jeon Ha-kyu said.

Police retrieved the leaflets in Paju and Ilsan, Gyeonggi Province, located just northwest of Seoul. 

About dozens of thousands of such North Korean leaflets were collected on Wednesday alone, one day after the North started sending the leaflets, the JCS spokesman said. 

"Our military is closely watching the situation," Jeon said, referring to growing security concerns.

Animosity is increasing across the border following North Korea's surprise nuclear test on Jan. 6. The country said it has successfully tested a hydrogen bomb, a claim that the outside world doubts because of the weak intensity of the earthquake registered by the detonation.

Two days later, Seoul started loudspeaker broadcasts along the tensely guarded border, blaring messages critical of North Korean leader Kim Jong-un in a resumption of the propaganda warfare that both countries had agreed to end in 2004.

The broadcasts are believed to infuriate the North, where the sacred image of the Kim family is maintained through tight control of outside information.

In counteractions, the North started its own propaganda broadcasts along the border and sent what was suspected as a surveillance drone across the border Wednesday.

South Korea fired about 20 warning shots before the aerial vehicle returned to the North.

South Korea is reviewing the addition of another psychology warfare tactic along the border, electronic display boards emitting political messages toward North Korea, Defense Ministry spokesman Kim Min-seok said in a briefing.

"A related plan is under review," Kim said. 

The resumption of the electric board operation would revive two of the three major propaganda operations, including the leaflets, which the two Koreas had actively employed in an ideological rivalry before the 2004 agreement brought them to a halt.

In a televised national address on Wednesday, President Park Geun-hye vowed to strengthen its psychological warfare against North Korea, calling it "the surest and most effective" measure.

Amid the growing tensions, Army Chief of Staff Gen. Jang Jun-gyu visited a front-line military division earlier in the day to inspect an artillery fire exercise. 

"Stand with full combat readiness so you can respond immediately if the enemy makes additional military provocations," the general told troops stationed there. (Yonhap)