North Korea sent 'propaganda' across border: Seoul

North Korea dropped suspected anti-South Korean "propaganda" into border areas overnight, Seoul's military told AFP on Wednesday, with one province issuing an alert asking residents to stay indoors.

Seoul's Joint Chiefs of Staff said that "unidentified objects believed to be North Korean propaganda leaflets have been identified in the Gyeonggi-Gangwon border area and the military is taking action".

"Citizens should refrain from outdoor activities, do not come into contact with any unknown objects, and report them to the nearest military base or police," it said in a statement sent to AFP.

Late Tuesday night, Gyeonggi province issued a text message alert to residents.

"Refrain from outdoor activities and report (objects from North Korea) to military bases when identified," it said in the message.

Since the 1950-53 Korean War ended in an armistice, the two Koreas remain technically at war and are separated by a heavily fortified border including the Demilitarised Zone (DMZ).

South Korean activists sometimes release balloons carrying anti-Kim Jong Un regime propaganda leaflets and money intended for people living north of the border.

Pyongyang has long been infuriated by such propaganda campaigns, possibly due to concerns that an influx of outside information in the tightly controlled society could pose a threat to the Kim regime.

On Sunday, North Korea warned it would retaliate in a statement carried by the official Korean Central News Agency.

"Tit-for-tat action will be also taken against frequent scattering of leaflets and other rubbish by the ROK near border areas," Kim Kang Il, a vice-minister of defence, said in the statement, using the acronym for South Korea's official name.

"Mounds of wastepaper and filth will soon be scattered over the border areas and the interior of the ROK and it will directly experience how much effort is required to remove them," Kim said.

North Korea on Monday attempted to put a second spy satellite into orbit, but the launch ended in a mid-air explosion.