North Korea sent a small group of soldiers to border posts for clearing bushes and road maintenance, according to South Korean government.

Pyongyang sent small groups of up to 5 soldiers each, carrying shovels, sickles and secret boxes to a number of border posts in the Demilitarized Zone (DMZ) separating the two regions.

In an angry reaction to South Korean media, North Korea threatened to redeploy troops to a joint industrial complex in the border city of Kaesong and the mountain resort of Kumgang on its east coast.

"The guard posts and secret boxes are all military service facilities," the source said, speaking on condition of anonymity.

South Korean military officials believe that the actions of a force from at least the platoon level in North Korea can be considered an abnormal sign, requiring special attention.

However, the source added that South Korea has yet to detect any action regarding the restoration of the guard posts.

The South Korean military is also closely monitoring two cannons along the coast in North Korea's Gaemori border area because the gun hatches are open, fearing they could be used at any time.

"But North Korea often opens and closes the cannon barrel like that. It is as if they open the barrel for dehumidification or ventilation," the source said.

Pyongyang has recently taken steps to exert pressure on Seoul, such as sending troops closer to the border and reestablishing guard posts in the Demilitarized Zone (DMZ), in response to a drop of leaflets to the north of

North Korea also declared severing ties and regarded Korea as an enemy.

The South Korean government in early June said it would ban the release of leaflets to the north, and filed a criminal complaint against two groups of North Korean defectors.

Relations between North Korea and South Korea have worsened since the nuclear talks between Pyongyang and Washington were at a standstill.