Three North Korean soldiers have defected to South Korea across the heavily-fortified border since August, media reports said Tuesday, in an apparent embarrassment to Pyongyang's new regime.

The rare defections -- involving one officer who shot dead two superiors before defecting on Saturday -- prompted the North to launch special probes on soldiers stationed at the border, Yonhap news agency and other media said.

They cited unnamed Seoul officials and the South's military chief Jeong Seung-Jo who had testified to lawmakers of the parliamentary defence committee in a closed meeting on Monday.

One soldier crossed the western part of the heavily-militarised border in August while another escaped in the eastern region on October 2, the reports said.

"Given their statements, the North's military appears to have serious lax discipline issues these days," said a Seoul official quoted by Yonhap.

Worsening food shortage at military barracks in the impoverished North has fanned frustration among its soldiers, the news agency quoted a South Korean intelligence official as saying.

The defence ministry declined to comment. Kim Kwang-Jin, a Seoul lawmaker and a member of the defence committee, confirmed the defection on October 2 without elaborating.

Military defections across the land border between the two Koreas are rare, with the last reported crossing by a North Korean soldier in 2010, and previous instances in 2008 and 2002.

North Korean soldiers stationed near the border are reportedly selected from families known to be the most loyal to the communist regime, to try to prevent military defections that may embarrass Pyongyang.

More than 23,500 North Koreans have escaped and resettled in the South since the end of the Korean War, but virtually all cross the North's border with China and most travel on to Southeast Asia in the hope of eventually reaching Seoul.

They face repatriation if discovered in China.

The isolated North has tightened border security-- particularly the increasingly porous frontier with China-- since the new leader Kim Jong-Un took over from his late father and longtime ruler Kim Jong-Il last December.

Kim Jong-Un visited the country's spy agency at the weekend and urged security agents to remain "loyal to the party to the last in any storm and stress" and to maintain "unshakeable faith."